The impending battle over replacing Antonin Scalia on the Supreme Court will undoubtedly loom over this year’s presidential contest. But it may have an even larger role in an often overlooked 2016 election contest: the fight for control of the Senate.

The Senate is in play this November, and the same vulnerable Republicans whose defeats might cost the G.O.P. control of the chamber are at once among the likeliest to back President Obama’s nominee. They are also the likeliest to suffer if the fight has political costs to the party.

The Democrats aren’t favored to retake the Senate. They would need to gain five seats (or four if they retain the presidency). But they have a real opportunity to win because a large number of Republicans from competitive or Democratic-leaning states are up for re-election. These Republican senators could have strong electoral incentives to support Mr. Obama’s Supreme Court nominee — otherwise, their opposition will be used against them.

The large number of relatively moderate Republicans from relatively moderate states is an artifact of the sweeping Republican victory in the midterm elections six years ago. In that election, Republicans won six Senate seats in states that Mr. Obama would carry in 2012: Ohio, Florida, Pennsylvania, Wisconsin, New Hampshire and Illinois.