The panel, asked for its preferences in collaboration with CBS and The New York Times, is unusual in its scope: It comprises more than 66,000 people living in states with a Senate race this year. YouGov, a polling firm, also conducted online interviews with roughly half of the panelists in previous years, allowing it to know how they voted in the past.

With the addition of the YouGov estimates to our model, the overall outlook for the Senate remains roughly the same. The Republicans appear to have a slight advantage, with the most likely outcome being a Republican gain of six seats, the minimum they need to finish with a 51-to-49-seat majority. But we, like many other forecasters, would not be surprised by a gain of anywhere from four to eight seats.

Summing up the possible outcomes, our model gives the Republicans a 60 percent chance of taking control, up from 54 percent on April 1.