Behind the Math: Why FiveThirtyEight predicts a GOP takeover in the Senate

Rick Klein, Olivier Knox, Richard Coolidge and Jordyn Phelps
Power Players

Top Line

Republicans are likely to win control of the Senate in November’s midterm elections, according to projections by the statistical prognosticators at FiveThirtyEight.

“We’re currently projecting that Republicans have a better chance than Democrats to control the Senate, but it's still up for grabs,” FiveThirtyEight political analyst Harry Enten said. “The current number that we're going for is a 62.2 percent chance that Republicans will take control of the United States Senate in November.”

Enten sat down with “Top Line” to discuss FiveThirtyEight’s Senate forecast, which has been updated since this interview’s taping Wednesday, and explained why it tells a story of GOP victory.

“We’re calculating the percentage chance that Republicans will win each seat, and then we're adding up those probabilities, and then we come up with an overall top line,” he said. “We believe that Republicans do have a better chance of controlling the Senate than Democrats.”

The projections for each race, Enten explained, don’t have to do with how many percentage points a given candidate leads their opponent; instead, it has to do with how consistent that lead is.

“Remember, President Obama only had a small lead through most of the 2012 campaign, but it was consistent, so the percentage chance of winning was higher,” Enten said.

And he pointed to another 2012 reality that continues to ring true in FiveThirtyEight’s 2014 projections.

“If you look at the map where we're projecting Republicans to win seats from Democrats, these are all states where Mitt Romney won and most of them are states where Mitt Romney won handily in 2012,” Enten said.

In looking at the map, Enten said the Kansas Senate election, where Democratic candidate Chad Taylor recently pulled out of the race – and, in so doing, dramatically shifted the ground game – has undergone the most dramatic changes in projecting the race’s outcome.

But barring the dramatic change in Kansas, Enten said, the Arkansas Senate race has been the most dynamic and hard-to-predict contest.

“Arkansas was a state that we weren't quite sure [of], flipping back and forth, we've always had a Republican leaning, but now we have it over a 70 percent chance that Republicans take the seat,” he said.

Come election night, Enten said there are three states to watch that could keep race observers and political junkies up late into the night and even beyond: Alaska, Louisiana, and Georgia.

“Alaska, it's just the last state to close counting is usually slow, it could be close,” he said. “Louisiana and Georgia: Louisiana, if no candidate hits 50 percent in November, they go to a runoff in early December –that's almost certainly going to be the case. In Georgia, if no candidate hits 50 percent in November, it goes all the way to early January.”

To get a peek of the FiveThirtyEight Senate map for yourself, and to hear Enten’s story about the emails and phone calls he fields from campaigns who disagree with their projections, check out this episode of “Power Players.”

ABC News’ Ali Dukakis, Tom Thornton, Dick Norling and Mark Banks contributed to this episode.