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Senate Forecast: What Has Gone Wrong for G.O.P. Candidates?

The trend in the presidential race has been difficult to discern lately. President Obama has very probably gained ground since the conventions, but it’s hard to say exactly how much, and how quickly his bounce is eroding.

There are no such ambiguities in the race for control of the Senate, however. Polls show key races shifting decisively toward the Democrats, with the Republican position deteriorating almost by the day.

Since we published our initial Senate forecast on Tuesday, Republicans have seen an additional decline in their standing in two major races.

Two polls of Virginia published on Wednesday gave the Democrat, the former Gov. Tim Kaine, leads of 4 and 7 percentage points over the Republican, the former Senator George Allen. The FiveThirtyEight forecast model now gives Mr. Kaine roughly a 75 percent chance of winning the seat on the strength of the new polls, up from about 60 percent in Tuesday’s forecast.

The other problematic state for Republicans is Wisconsin, where their candidate, the former Gov. Tommy Thompson, had once appeared to hold the advantage.

Mr. Thompson’s Democratic opponent, Representative Tammy Baldwin, had published an internal poll earlier this week showing her pulling into the lead. The FiveThirtyEight Senate and presidential forecasts do not use internal polls released directly by the campaigns, as they typically exaggerate their candidate’s standing.

However, in this case, public polls have now confirmed that the race seems to have shifted. A poll by The New York Times, CBS News and Quinnipiac University showed Ms. Baldwin having drawn into a tie with Mr. Thompson, after trailing him by 6 percentage points last month.

A Marquette University poll, also published on Wednesday, showed a much sharper reversal, with Ms. Baldwin going from a 9-point deficit to a 9-point lead. The Marquette poll appears to be a bit of an outlier — it also had Mr. Obama leading in the presidential race in Wisconsin by a 14-point margin, a somewhat implausible figure. Nonetheless, the model now has Ms. Baldwin as the slight favorite, with about a 60 percent chance of winning.

It would be only a modest exaggeration to say that it’s been hard to find any strong Senate polls for Republicans in the past two or three weeks. Wednesday also brought bad news for Republicans in Massachusetts, where a fourth consecutive poll showed the Democrat Elizabeth Warren ahead of Senator Scott Brown; in Connecticut, where a poll gave the Democrat Chris Murphy a slight advantage over their candidate, Linda McMahon; and in Florida, where a Fox News poll gave the Democratic incumbent Bill Nelson a 14-point lead.

One exception has been in Maine, where two new polls on Wednesday showed a deterioration in the standing of the independent Angus King, who would probably caucus with the Democrats, at the expense of both the Republican, Secretary of State Charles Summers, and the Democrat, State Senator Cynthia Dill. The model now gives Mr. King an 84 percent chance of winning, Mr. Summers 11 percent, and Mrs. Dill 5 percent.

But this is small compensation for the decline over the past two weeks of the Republicans’ position in Virginia, Massachusetts, Wisconsin, Florida and Ohio, all of which have broken sharply to the Democrats.

The Democrats’ chances of controlling the Senate have increased to 79 percent in the forecast, up from 70 percent on Tuesday.

Had we run the model a month ago, based on polls through Aug. 19, the Democrats’ chances of maintaining Senate control would have been listed at just 39 percent.

The velocity of the change is unusual. Although Senate races in different parts of the country can sometimes move in the same direction, there was never quite this rapid a shift in our Senate forecasts in 2008 or 2010.

The forecast model is not doing anything particularly fancy; it’s just that an overwhelming number of Senate polls recently have shown the Democratic candidates’ standing improving.

Republicans could also have some reason to be concerned about Nevada, which has not been polled recently but where their candidate, the appointed Senator Dean Heller, maintains a slight advantage over the Democratic Representative Shelley Berkeley. Mr. Heller is a fairly strong candidate, but if there is some sort of national tide against Republicans, he could become the underdog in that race as well.

There’s one comforting thought for Republicans, however. It’s plausible that at least some of the Democrats’ gains reflect a boost from their convention, which could wear off. I have not studied whether conventions produce bounces in Congressional races and the model makes no assumptions about it, but it is a reasonable hypothesis.

However, at least some of the shift appeared to predate the conventions. The model would have had Democrats’ chances of retaining the Senate improving to 50 percent from 39 percent over the course of the week beginning Aug. 19, before the Republican convention started.

I can think of two major theories to explain why the shift is occurring, one focused on Mitt Romney, and another on the overall positioning of the Republican brand.

Theory No. 1: Is Romney a Downballot Drag?

Polls show that Mr. Romney has middling personal favorability ratings but that many voters will choose him anyway because of the deteriorating economy.

Senate races, however, are less dictated by national economic conditions. Instead, they often turn more on the strengths and weaknesses of the individual candidates, and then by their stances on fiscal and social policy.

Mr. Romney has not dictated much in the way of detailed programs in these areas, and some of the policy stances that he has articulated are unpopular.

Mr. Romney has also been less able to campaign effectively against an unpopular Democratic initiative, the Democrats’ health care bill, because he passed a similar bill as governor of Massachusetts.

Finally, some voters who disapprove of Mr. Obama, but who also have lukewarm feelings toward Mr. Romney, might lean toward voting Democrat for Senate in effort to ensure divided government, especially since Republicans also have control of the House.

Theory No. 2: G.O.P. Conservatism Is Hurting

An alternative hypothesis is that the shift has to do with overall perceptions of the Republican platform.

Our research has shown that statistical measures of candidate ideology are a reasonably powerful predictor of the outcome of Senate races, with candidates who are rated as holding “extreme” views performing poorly.

But in practice, ideology is in large part perceptual for voters, and may depend on which issues seem most salient at any given time.

August, at which point the shift toward Democrats in Senate races appeared to begin, was dominated by two major news items: Mr. Romney’s selection of Representative Paul D. Ryan as his running mate, who has very conservative views on fiscal policy, and by the comments about rap
e made by the Republican Senate candidate in Missouri, Todd Akin, which may have reinforced the idea that Republicans hold very conservative positions on social issues.

These factors may have made it harder for Republicans to position themselves toward the ideological center. And in several states, including Missouri, Indiana, Michigan and Ohio, Republicans nominated sub-optimally conservative candidates.

More moderate Republican candidates, like Mr. Brown of Massachusetts and Mrs. McMahon of Connecticut, have increasingly sought to distance themselves from the national Republican brand, and sometimes also from the Republican presidential ticket.

Some of these theories are speculative, to be sure. A large number of Senate races remain in play: of the several states in which there has been a shift against Republicans in the polls in recent weeks, perhaps only Florida seems completely lost.

But if the trend continues, the question may no longer be whether Republicans can win the Senate — but how vulnerable they are to losing the House.

Nate Silver founded and was the editor in chief of FiveThirtyEight.