64% CHANCE OF REBOOBLICANS TAKING THE SENATE.

NATE SILVER, FIVE BY THIRTY EIGHT

Republicans are favored to take the Senate, at least in our view; the FiveThirtyEight forecast model gives them a 64 percent chance of doing so.

The reasons for the GOP advantage are pretty straightforward.
Midterm elections are usually poor for the president’s party, and the Senate contests this year are in states where, on average, President Obama won just 46 percent of the vote in 2012.¹ Democrats are battling a hangover effect in these states, most of which were last contested in 2008, a high-water mark for the party. On the basis of polling and the other indicators our model evaluates, Republicans are more likely than not to win the six seats they need to take over the Senate. This isn’t news, exactly; the same conditions held way back in March.
An equally important theme is the high degree of uncertainty around that outcome. A large number of states remain competitive, and Democrats could easily retain the Senate. It’s also possible that the landscape could shift further in Republicans’ direction. Our model regards a true Republican wave as possible: It gives the party almost a 25 percent chance of finishing with 54 or more Senate seats once all the votes are counted.²

Why so much uncertainty? Consider some of the challenges that election forecasters face this year:

• The quality and quantity of polling has been poor. We have stunningly few polls in some states. In Colorado, for example, which could easily determine the balance of the Senate, no polls at all were published in August. And in many states, most of the polls we do have are from non-traditional polling firms, including those that conduct “robopolls” or Internet polls, or which have an explicitly partisan affiliation.

• In contrast to 2010, when major swing states like Pennsylvania, Ohio and Wisconsin were key to the Senate picture, this year’s races take us all over the country, including to politically idiosyncratic states like Arkansas and Alaska. These states are harder to pin down: Arkansas has begun to vote strongly Republican for president, for example, but sometimes still elects Democrats to other offices. In certain other states, polls and partisanship diverge. Ordinarily in a year like this one, you’d give Democrats no chance at all in Kansas, for example. But polls show a somewhat close race there, and there’s further uncertainty because of the presence of an independent candidate.

• Readings of the national mood are ambiguous. President Obama remains unpopular — and the president’s party has a long history of performing poorly in midterm election years. However, Democrats remain roughly tied with Republicans on the generic congressional ballot. That may reflect the Republican Party’s poor image, which remains quite a bit worse than that of the Democratic Party. But it could also mean there’s some upside for Republicans. On average in midterm years since 1990, the generic ballot has favored the opposition party by 5 percentage points by Election Day instead of being even.

• We don’t yet have a good sense for the potential turnout and enthusiasm advantage. It’s reasonably safe to assume it will benefit Republicans; their (older, whiter) demographics are usually associated with higher turnout at the midterms. But we don’t have much evidence yet about the magnitude of this effect. One way of measuring the turnout edge is to compare polls that release results among both registered and likely voters in the same state. In 2010, Republicans polled a net of 6 percentage points better in the likely-voter surveys, a historic high. Compare that with a GOP advantage of about 2 percentage points on average in midterms from 1990 to 2006. This year, however, we’ve seen very few polls to release both registered and likely-voter results in the same state. That leaves us somewhat in the dark about whether 2010’s turnout pattern was a fluke.

Another complication is the broad Senate playing field. There are, according to our model, either nine or 10 races in which each party has at least a 20 percent chance of winning.³ There are several others in which each party’s chances are at least 10 percent.
The Republicans’ edge comes from an abundance of opportunity. They are almost certain to win the Democratic-held seats in Montana and West Virginia, and very likely to do so in South Dakota. That gives them three of the six seats they need. Beyond that, they have few guarantees but a lot of good prospects:

• Republicans are slightly favored, though far from certain, to oust Democratic incumbents in Louisiana and Arkansas.
• Four more Democratic-held seats — in Alaska, Colorado, Iowa and North Carolina — rate as tossups.
• While Democrats are favored in Michigan and New Hampshire, Republicans retain some chances to win those states as well.

Democrats, by contrast, have plausible chances to win only three Republican-held seats — in Georgia, Kentucky and Kansas — and they aren’t favored in any of those races.

Footnotes:

1. This calculation excludes states holding special Senate elections. ^
2. What about Republicans’ dream of a 60-seat, filibuster-proof majority? The model gives them only a 0.1 percent chance of that. ^
3. The ambiguous case is Kansas, where the odds for Democrats are 17 percent, for Republicans 79 percent, and for the independent candidate Greg Orman 3 percent. ^