One key piece of information is whether candidates have held an elective office before and, if so, which one. Unsurprisingly, political science research has long shown that candidates who have held elective office and higher levels of office tend to do better on Election Day. They usually run better campaigns and make fewer mistakes, if only because they’ve done it before.

As we have begun to incorporate candidate experience into the model, our initial sense is this: Republicans may have a far better chance of winning control of the Senate than we or other analysts previously thought. Here is a preliminary estimate: The GOP could have as much as a 4 in 5 chance of controlling the chamber…

Democrats need 50 seats to control a majority, given Vice President Biden’s tie-breaking vote. This occurs in only 18 percent of simulations. It is most likely that Democrats will control 46 to 49 seats. Indeed, in nearly one-third of simulations, Democrats control 48 or 49 seats, suggesting that if future events break in their favor — for example, President Obama becomes more popular — their chances of controlling a narrow majority could improve.