John Sides, Ben Highton and Eric McGhee of The Monkey Cage have started Election Lab, a Senate forecasting model at The Washington Post. Their model gives Republicans more than an 80 percent chance of retaking the Senate; that’s much more than the 54 percent chance that Leo, The Upshot’s Senate model, gives the G.O.P.

It’s also better odds than many Republican strategists give their party. In a recent survey by Reid J. Epstein of The Wall Street Journal, Kevin Madden, a former aide to Mitt Romney, gave the party a 65 percent chance of taking the Senate. John Brabender, Rick Santorum’s former campaign manager, put the odds at 75 percent.

As we’ve written before, we plan to tell you not only about the assumptions behind The Upshot’s Senate model but also to explain why and how it differs from others. So what explains the difference between Election Lab and Leo?