Based on the 2016 presidential results, Sens. Cory Gardner of Colorado and Susan Collins of Maine would be the most vulnerable senators, as the only two Republicans in states that Clinton carried in 2016.

There won’t be any shortage of Democrats looking to take on Gardner, particularly after his vote to confirm Justice Brett Kavanaugh and his cycle as chairman of the National Republican Senatorial Committee. And Democratic donors around the country rallied to raise more than $4 million for a Collins challenger in the wake of the Kavanaugh hearings.

Sens. Joni Ernst of Iowa and Thom Tillis of North Carolina represent Trump states that Obama carried previously and should be regarded as initially vulnerable as well. Trump and Romney carried Arizona, but uncertainty surrounding the late John McCain’s seat could make it interesting. The seat will be on the ballot in November 2020.

Republican retirements are also a possibility. Sen. Pat Roberts of Kansas is 82 years old and faced a competitive primary in 2014, which he won 48 percent to 41 percent. Sen. Jim Inhofe of Oklahoma is 83 years old, but doesn’t appear to be going anywhere. Competitive primaries are likely if either or both seats open up.