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Senate Update: Five Races Were On A Knife’s Edge Even Before Comey’s Letter

The fight for the Senate remains quite close. Democrats have a 65 percent chance of taking control, according to our polls-plus model and a 62 percent chance according to polls-only. But it’s not just that the overall race for a Senate majority is close. There is an unusually high number of close races. That’s one reason the latest reports about Hillary Clinton’s private email server could be important — even a small shift could have a big effect in a lot of races.

There are five Senate races in which the FiveThirtyEight polls-plus model puts the margin separating the two major-party candidates within 2 percentage points: Missouri, Nevada, New Hampshire, North Carolina and Pennsylvania. All five of these races have remained close throughout October, with no sign of either candidate breaking away. If the model perfectly projects all five of these races, 2016 would be only the second election in the past 35 years in which more than three Senate races had final margins of 2 points or less.

2016* 5
2014 3
2012 2
2010 2
2008 2
2006 2
2004 3
2002 2
2000 2
1998 2
1996 2
1994 1
1992 2
1990 0
1988 3
1986 7
1984 2
1982 1
2016 could have an unusually high number of close Senate races

* 2016 margins determined by FiveThirtyEight polls-plus forecast. All other years years are based on the final results

Source: Dave Leip’s US Election Atlas, Federal Elections Commission

Why are there so many tight races this year? First, and most simply, there are a lot of races in purple states. Four of the five races mentioned above, for example, are taking place in presidential battleground states. And even though Donald Trump is down by about 5 percentage points nationally, Republican candidates for Senate are running 4.5 percentage points ahead of Trump, on average.

Additionally, Trump’s candidacy has made the map more “compressed” — more states are competitive than in 2012. The higher number of battleground states on the presidential level is reminiscent of a map we would have seen before the high level of polarization we’ve grown accustomed to in recent years. Perhaps not coincidentally, the last time we saw so many tight Senate races was 30 years ago.

These five close races — along with Indiana, where Democrat Evan Bayh is in an increasingly tight race against Republican Todd Young — are very likely to determine Senate control. If Clinton wins the White House, Democrats need a net gain of four seats (that gets them to a 50-50 tie, which Tim Kaine would break). Democrats already hold an advantage of 3 percentage points or more in Illinois and Wisconsin, both of which have Republican incumbents. That’s two. So then Democrats just need to win two more seats among Indiana, Missouri, New Hampshire, North Carolina and Pennsylvania, all of which are Republican-controlled, and hold onto Democrat Harry Reid’s seat in Nevada.

But all these races are so close that a small shift — or a polling error — could swing things. A 2-percentage-point lead just isn’t that safe, particularly in light of the latest Clinton email reports. My partial list of “October surprises” shifted presidential polls between 1 and 2 percentage points, on average. Further, FBI Director James Comey’s announcement in July that Clinton would not be criminally charged in the email case — in which Comey called Clinton “extremely careless” — preceded a 2-point drop in Clinton’s margin over Trump.

Of course, Clinton might not lose support this time around. And even if she does, down-ballot Democrats might not be affected. But there are a few reasons they could. First, we’ve seen that when Clinton’s numbers have gone done this year, so too have Democratic Senate candidates’. Second, there’s been an increasingly strong relationship between Senate and presidential election results. Third, some evidence exists that people are more prone to split their tickets when they have some certainty about who is going to win the presidency. Given Clinton still has a semi-comfortable lead in the presidential race, it’s not difficult to imagine that the latest news might push a few more voters to want a Republican Congress as a check on Clinton.

We could easily end up with a situation like we had in 2006 when it wasn’t clear who was going to control the Senate until the day after the election. That year, moreover, the majority of Senate races broke late for Democratic candidates. The reverse happened in 2014, with Republicans beating their polls in most states. That could happen this year. We’ll have to wait and see what, if any, impact the resurgence of the Clinton email story has. And even if it doesn’t change the polls, control of the Senate looks like it will go down to the wire.

Harry Enten was a senior political writer and analyst for FiveThirtyEight.