We last issued a U.S. Senate forecast in mid-March. Not a lot has changed since then.
The Senate playing field remains fairly broad. There are 10 races where we give each party at least a 20 percent chance of winning,1 so there is a fairly wide range of possible outcomes. But all but two of those highly competitive races (the two exceptions are Georgia and Kentucky) are in states that are currently held by Democrats. Furthermore, there are three states — South Dakota, West Virginia, and Montana2 — where Democratic incumbents are retiring, and where Republicans have better than an 80 percent chance of making a pickup, in our view.
So it’s almost certain that Republicans are going to gain seats. The question is whether they’ll net the six pickups necessary to win control of the Senate. If the Republicans win only five seats, the Senate would be split 50-50 but Democrats would continue to control it because of the tie-breaking vote of Vice President Joseph Biden.
Our March forecast projected a Republicans gain of 5.8 seats. You’ll no doubt notice the decimal place; how can a party win a fraction of a Senate seat? It can’t, but our forecasts are probabilistic; a gain of 5.8 seats is the total you get by summing the probabilities from each individual race. Because 5.8 seats is closer to six (a Republican takeover) than five (not quite), we characterized the GOP as a slight favorite to win the Senate.