Democrats are now favored to retain control of the Senate when the new Congress convenes in January, according to the FiveThirtyEight forecast, breaking a summer stalemate during which control of the chamber appeared about equally likely to go either way.
An unusually large number of Senate races remain competitive, meaning that a wide range of outcomes are still possible. Republicans have about a 10 percent chance of winning a net of at least six seats from Democrats, according to the forecast, which would give them control of at least 53 seats next year. However, there is also about a 20 percent chance that Democrats could actually gain Senate seats on balance, giving them at least 54. The only thing that seems completely assured is that neither party will control enough seats next year to hold a filibuster-proof majority.
But the odds of a favorable overall outcome for Democrats have increased in recent weeks. The forecast model now gives them a 70 percent chance of controlling the chamber, either by having at least 50 seats and the presidency, or 51 without it.
Although this represents the first official FiveThirtyEight forecast for the Senate this year, I ran backdated forecasts to July 1 based on the polls that were available at that time. Two weeks ago, for example, the model would have given Democrats a 52 percent chance of retaining Senate control — and four weeks ago, it would have given them a 39 percent chance.
The trend toward Democrats is a relatively recent one. Part of the shift may reflect the bounce President Obama received from the Democratic convention. If so, it could recede, especially if Mr. Obama’s poll numbers do so, too.
But our analysis also suggests that the Democratic advantage has probably been building over the past few weeks, and may not have any one root cause. Instead, Republicans risk death by a thousand cuts, with a gradual deterioration in their standing in several important races, and their inability to field optimal candidates in others.