Live Coverage of the South Carolina Primary

With nearly all votes counted in South Carolina, Mr. Gingrich’s margin of victory is about 75,000 votes, or 12 percentage points over Mitt Romney.

Turnout in the contest was about 600,000 voters, a significant increase from the roughly 450,000 who voted in the state in 2008.

Rick Santorum finished in third place with 17 percent of the vote.

How did our forecasts do?

FiveThirtyEight projected Mr. Gingrich to win 39 percent of the vote; he got 40 percent. Mr. Romney, whom we projected to get 29 percent, got 28 percent. We were slightly low on Rick Santorum, who got 17 percent rather than his projection of 14 percent, and slightly high on Ron Paul, who got 13 percent rather than 17 percent.

Overall, I’d consider this a pretty fortunate performance for the model, given how volatile the polling was.

— Nate Silver

I just posed the following question to my Twitter readers:

What are Mitt Romney’s chances of winning G.O.P. nomination if (1) Romney wins Florida or (2) Romney loses Florida?.

With more than 50 votes counted, the average response was that Mitt Romney has an 87 percent chance of winning the Republican nomination if he wins Florida, but only a 53 percent chance if he doesn’t.

— Nate Silver

It’s worth noting that, ultimately, the Republican candidates are not really competing for state victories, or even votes. In the end, the Republican nomination will be decided by delegates.

Mr. Gingrich has so far secured 19 delegates in South Carolina, and he will probably gain at least a few more when all the votes are counted.

The New York Times delegate counter currently has Mitt Romney and Mr. Gingrich tied at 19 delegates each. Most likely, after tonight, Mr. Gingrich will be leading the delegate race to the Republican nomination. Of course, a candidate needs 1,144 delegates to secure the nomination, so that race has just begun.

— Micah Cohen

In both the Iowa Republican caucuses and the New Hampshire Republican primary, turnout was higher than it was in 2008. However, the increase was slight in both instances, and it was explained largely by the fact that more independents and Democrats turned out to vote in the Republican contest because there was no competitive Democratic race. Among registered Republicans, turnout was down in both states.

The same is not true in South Carolina. More than 520,000 votes have already been counted, with 12 percent of precincts still outstanding. Exit polls show that 71 percent of these votes were cast by self-identified Republicans, or about 370,000 total.

By contrast, in 2008, there were about 446,000 votes in South Carolina, 80 percent of which were cast by Republicans, or about 357,000 total.

Thus, the G.O.P. has already clinched an overall turnout increase in South Carolina. Once all votes are in, turnout may be up roughly 30 percent overall from 2008, and by 15 to 20 percent among Republicans.

Perhaps there is something to the notion that a conservative candidate like Newt Gingrich can do more to motivate the Republican base to vote.

— Nate Silver

I mentioned earlier tonight that polls showing Mitt Romney with a large lead in Florida need to be interpreted carefully, since they were conducted before Mr. Gingrich’s surge in national and South Carolina polls over the past week. During this time, Mr. Gingrich has gained somewhere between 15 and 25 points on Mr. Romney by different measures, roughly matching Mr. Romney’s lead in the Florida polls.

Now, Mr. Gingrich has additional momentum after his clear win in South Carolina tonight.

The betting market Intrade, making inferences in the absence of polling, now gives Mr. Romney about a 60 percent chance of winning Florida and Mr. Gingrich a 40 percent chance.

That looks roughly correct to me. My guess is that if Mr. Romney and Mr. Gingrich are roughly tied in the Florida polls once we get fresh data, Mr. Romney would be the slight favorite to win the state because of his advantages in advertising dollars and early voting. And the bounce from winning a primary can sometimes fade over a matter of days.

However, given his big win tonight, it is also possible that Mr. Gingrich will lead Mr. Romney once new polls are released there. If he starts out with a cushion of three to five points over Mr. Romney, for instance, his position there will look more robust.

— Nate Silver

Newt Gingrich isn’t the only 2012 candidate seemingly happy with the results in South Carolina tonight. David Axelrod, the longtime campaign strategist for President Obama, sent this Twitter post about Mitt Romney to FiveThirtyEight:

The answer, Mr. Axelrod, is a lot.

— Micah Cohen

Although Mitt Romney finished far ahead of Newt Gingrich in both Iowa and New Hampshire, those states are less populous than South Carolina, where Mr. Gingrich won big tonight.

Mr. Romney got 13,642 more votes than Mr. Gingrich in the Iowa caucuses, and 74,121 more votes in the New Hampshire primary, giving him an 87,763-vote advantage heading into tonight.

However, Mr. Gingrich leads Mr. Romney by more than 65,000 votes in South Carolina, with 80 percent of precincts reporting. If the last 20 percent of precincts come in especially strongly for Mr. Gingrich, he could beat Mr. Romney by about 80,000 votes in the state, making the overall popular vote very close.

— Nate Silver

I’ve seen some comparisons made between Newt Gingrich’s comeback tonight and Hillary Rodham Clinton’s in the New Hampshire primary in 2008. By one measure, however, Mr. Gingrich’s comeback is twice as impressive.

Mrs. Clinton trailed Barack Obama in New Hampshire by roughly eight points in New Hampshire polls in 2008, but wound up winning the state by three points — representing an 11-point swing.

By contrast, Mr. Gingrich trailed Mitt Romney by 12 points in FiveThirtyEight’s polling-based forecast before Monday night’s debate. By this afternoon, our model had shifted to project a 10-point win for Mr. Gingrich, and his actual lead over Mr. Romney is 14 points based on the votes counted so far.

If his current lead holds, it would represent a total swing of 26 percentage points in Mr. Gingrich’s direction — from a 12-point deficit in the polls just days ago to a 14-point win — more than twice as large as Mrs. Clinton’s.

— Nate Silver

Correction Appended

South Carolina’s delegate allocation rules award 11 delegates to the statewide winner — declared by The New York Times and other organizations to be Newt Gingrich.

The state then awards two delegates to the winner in each of its
seven Congressional districts View on Map), where Mr. Romney has an extremely narrow lead over Mr. Gingrich so far, and in a few counties surrounding the state capital, Columbia, where the same holds true.

Doing just slightly better in Columbia might not be enough to help Mr. Romney pick up any delegates, since the metro area is split among several different Congressional districts.

Mr. Romney’s best chance to win delegates might instead be in the Seventh Congressional District, which includes Charleston.

In total, Mr. Gingrich is likely to win at least five of seven Congressional districts tonight, which would give him at least 21 of the state’s 25 delegates. And he could get a clean sweep.

Correction: A previous version of this post mis-stated the name and county makeup of one of South Carolina’s Congressional districts. The state’s delegates will be awarded based on South Carolina’s new Congressional districts following its recent redistricting. The district which includes Charleston and where Mr. Romney is most competitive is now labeled the 7th Congressional district rather than the 1st Congressional district. In addition to Charleston County, it includes Beaufort County rather than Georgetown County.

— Nate Silver

With about a fifth of precincts reporting in South Carolina, Stephen Colbert’s presidential campaign — a not-so-serious campaign with a serious message — appears to be struggling.

Mr. Colbert had urged South Carolinians to vote for Herman Cain as a Colbert proxy (Mr. Cain was already on the ballot, while Mr. Colbert wasn’t). But so far, Mr. Cain has under 1 percent of the vote.

— Micah Cohen

With 18 percent of the vote counted in South Carolina, Rick Santorum has 17 percent of the vote to Ron Paul’s 13 percent. Although it is not yet safe to conclude that Mr. Santorum will beat Mr. Paul for third, Mr. Santorum also led Mr. Paul in exit polls.

How much difference this will make is hard to say. As I wrote earlier today, Mr. Santorum’s best hope of becoming a factor in the race is a decision by influential Republicans that he could be a potential compromise choice between Newt Gingrich and Mitt Romney.

— Nate Silver

In 2008, Georgetown County ( View on Map) , which is just southwest of Myrtle Beach, was one of Mr. Romney’s best in South Carolina. He won 20 percent of the vote there, more than in all but two other counties in the state.

Tonight, Georgetown County is ahead of most others in the state, with about 1,500 votes already having been reported among eight precincts. Mr. Romney got 34 percent of those votes, an improvement from his showing in 2008. But Mr. Gingrich is doing better than Mr. Romney there, with 43 percent.

— Nate Silver

Although Florida has a more moderate Republican electorate than South Carolina, that difference alone would not be enough to hand a victory to Mr. Romney there, extrapolating from the vote in South Carolina tonight.

According to tonight’s exit polls from South Carolina, 37 percent of voters describe themselves as very conservative, 32 percent as somewhat conservative and 32 percent as moderate or liberal.

By contrast, in Florida in 2008, 27 percent of Republican primary voters said they were very conservative, 34 percent somewhat conservative and 38 percent moderate or liberal.

Mr. Gingrich holds a solid lead over Mr. Romney among both very conservative and somewhat conservative voters in South Carolina tonight, according to the exit polls. Mr. Romney does have the edge among moderates, but only by five percentage points (35 percent to 30 percent).

In fact, if you weight tonight’s South Carolina vote to resemble Florida’s ideological dispositions from 2008, Mr. Gingrich would still win there by 37 percent to 29 percent.

There are other factors working in Mr. Romney’s favor in Florida, however, like the fact that many voters already cast ballots there before his recent slide in the polls.

— Nate Silver

Our first reader comment highlight of the night comes from Jeff Alexander in New York City:

Last week the narrative was: no non-incumbent had ever won IA and NH. Tonight, we will hear over and over again how never before have three different people won IA, NH, and SC.

Jeff is right — as far as Republicans are concerned. In the modern primary system (since 1972), in years without an incumbent Republican president running, never before have three separate candidates claimed victory in Iowa, New Hampshire and South Carolina.

John McCain won New Hampshire and South Carolina in 2008, George W. Bush won Iowa and South Carolina in 2000, Bob Dole won Iowa and South Carolina in 1996, George H. W. Bush won New Hampshire and South Carolina in 1988 and Ronald Reagan won New Hampshire and South Carolina in 1980. Republican presidents were all running for re-election in 2004, 1992, 1984, 1976 and 1972 (although Gerald Ford was an incumbent trying to win his first presidential election in 1976).

— Micah Cohen

According to exit polls, only 28 percent of voters in South Carolina’s primary said they took a generally negative view of Mitt Romney’s “background of investing in and restructuring companies,” while 65 percent said they took a generally positive view.

There was, however, a substantial difference in how people voted based on their responses to this question. Among those who said they had a positive view of Mr. Romney’s business experience, Mr. Romney led Newt Gingrich 41 percent to 31 percent. But only 3 percent of voters who had a negative view went to Mr. Romney, while 48 percent went to Mr. Gingrich.

But be careful if you see other analysts suggest that this was a critical factor in Mr. Gingrich’s win. Often, when questions like these are posed to voters, they react based upon their overall impression of the candidates rather than the specific issue. Those voters who were loyal to Mr. Gingrich to begin with, for instance, probably took a more negative view of Mr. Romney’s business activities, since Mr. Gingrich emphasized this line of attack. Put another way: correlation is not proof of causation.

— Nate Silver

With the tightening of the Republican race over the past several days, it’s probably worth taking a look at the primary calendar. Florida, as most of you will know, votes next. Then there is a lull in which there are no primaries for nearly a month, although there are caucuses in Nevada and other states.

Michigan and Arizona vote on Feb. 28, and they are likely to get a lot of attention if the race remains competitive up to that point.

Then comes Super Tuesday on March 6, when 11 states are set to vote.

But much of the attention on Super Tuesday is likely to boil down to a single state: Ohio. The reason is that two of the larger states to vote that day, Georgia and Massachusetts, are the home states of Newt Gingrich and Mitt Romney, respectively. Virginia, meanwhile, will not have any candidates but Mr. Romney and Ron Paul on its ballot; others, like Mr. Gingrich, failed to collect enough signatures to qualify.

Most of the other states participating in Super Tuesday are small. There are caucuses in Alaska, Idaho, North Dakota and Wyoming and primaries in Oklahoma, Tennessee and Vermont.

That leaves only Ohio. Thus, it could be that the two classic swing states in general elections — Ohio and Florida — could prove to be the most critical in the Republican nominating contest as well.

— Nate Silver

Significantly more voters seem to have decided whom to vote for in the waning days of this year’s South Carolina campaign, compared with previous elections.

This cycle’s high number of late deciders is particularly striking when compared with the state’s Republican voters in 2000, most of whom made up their minds before the closing days of the campaign.

— Micah Cohen

Since 1980, every Republican winner in South Carolina has gone on to win the party nomination.

If Newt Gingrich prevails over Mitt Romney tonight, he might represent a challenge to that precedent. At the betting market Intrade, bettors now give Mr. Gingrich a 95 percent chance of winning South Carolina, but only a 25 percent of winning the nomination (although that figure is up dramatically in recent days).

On the other hand, it’s not clear how meaningful the precedent was to begin with. In Republican races from 1980 to 2008, there was a reasonably clear national front-runner in the polls after New Hampshire. South Carolina did go with the front-runner rather than buck the trend, but most other states would have done the same thing.

Sometimes, in fact, South Carolina gave the front-runner a close call. In 2008, John McCain had roughly a nine-point lead over Mike Huckabee in national polls but beat him in South Carolina by only three points. In 2000, George W. Bush held a 26-point national lead over John McCain but beat Mr. McCain by only 12 points in South Carolina and only after a late surge in the polls.

South Carolina also did once pick the “wrong” candidate in the Democratic side. In 2004, John Edwards won the primary there, rather than the front-runner and eventual nominee, John Kerry.

— Nate Silver

Florida is a reasonably strong state for Mitt Romney. But it’s not clear how large his lead is there, even before accounting for anything that might happen tonight.

FiveThirtyEight’s current forecast of Florida shows Mr. Romney with 46 percent of the vote there to 25 percent for Newt Gingrich — a 21-point lead.

Essentially all of the polling data used for the forecast, however, predates the Monday night debate in Myrtle Beach; since then, there has been a dramatic reversal of fortunes in the Republican race. Mr. Romney lost 15 points off his national polling lead in the Gallup national tracking poll over this period. There has been an even larger swing — a net of about 21 points between Mr. Romney and Mr. Gingrich — in South Carolina.

If the Florida polls have swung as much as the national polls during the past several days, Mr. Romney would have only about a five-point lead there now. And if the Florida polls have swung as much as the South Carolina polls have during the last week, Mr. Gingrich and Mr. Romney would now be essentially tied.

— Nate Silver

According to early exit polls, 37 percent of voters in tonight’s South Carolina primary describe themselves as “very conservative.”

That proportion is only slightly higher than in 2008, when 34 percent of voters identified themselves the same way. But it’s significantly higher from 2000, when 24 percent of voters said they were very conservative, or 1996, when 25 percent did.

— Nate Silver