Republicans have a 53-47 majority in the Senate that they’re trying to hold on to in November, while Democrats hope to take control with a net gain of four Senate seats. If they win three and the White House, they also gain control because the vice president can break any ties.

The seats in play: Two Senate seats in Georgia and 33 others are on the ballot. Twelve are controlled by Democrats, while 23 are held by Republicans.

Most likely to flip:

Alabama: Democratic Sen. Doug Jones is considered the most vulnerable senator in 2020. He’s facing former Auburn University coach and first-time candidate Tommy Tuberville.

Colorado (Republican-held): Republican Sen. Cory Gardner is trying to fend off a strong challenge from former Gov. John Hickenlooper.

Arizona (Republican-held): Republican Sen. Martha McSally, who was appointed in December 2018 to fill the seat vacated by interim Sen. Jon Kyl, is running against former astronaut and gun-control activist Mark Kelly.