Who Will Win The Senate?

For months, we’ve been tracking the probability of each party controlling the Senate after the 2014 elections. Here’s how the chances have changed over time.

Below, a compilation of the latest shifts in Senate race ratings — both in our model and in others.

To forecast each party’s chance of gaining a majority, our model first calculates win probabilities for each individual Senate race. In addition to the latest polls, it incorporates the candidates’ political experience, fund-raising, a state’s past election results and national polling. More about our methodology.

With these state-by-state probabilities in hand, our machine randomly simulates all 36 Senate elections. Give it a try below. The spinners are calibrated to the current probabilities we’ve calculated for each state. We let the states move together to some extent, but you’ll no doubt occasionally see some surprising results — over 25 percent of the time, at least one of the races we call “likely” will be won by the opposite party.

We run the above simulation 250,000 times and tally the results. (Don’t worry, we don’t do it by hand.) The table below shows the outcomes of those simulations. By counting up the simulations that resulted in a Republican majority, we can estimate the probability that they will win a majority in the Senate. Likewise for the Democrats.1

Several other organizations perform similar calculations. Some use similar statistical models; others rely on reporting and knowledgeable experts’ opinions. We compile and standardize them every day into one scoreboard for comparison. View all states.

What are the odds of “overtime” — that Senate control hinges on a December runoff in Louisiana? Or that the elections leave Republicans in control of 54 seats or more? In addition to which party controls the Senate, here are some other interesting outcomes worth following.

Based on incumbent winning percentages in previous Senate elections, we’d expect about three incumbents to lose their seats this year – these are the five most likely to fail to be re-elected.

We let you explore the effect of different assumptions and submit a new forecast every day. Create and share your own forecast.