Emboldened by their electoral prospects in November, Democrats are planning to redouble their efforts to make the fate of the Supreme Court a signature election issue, with the Democratic leader in the Senate threatening to stall Republican legislative priorities if no action is taken on the confirmation of Judge Merrick B. Garland.

The Senate has been stuck in a stalemate since the death of Justice Antonin Scalia in February left a vacancy on the bench. Republicans have refused to hold confirmation hearings on President Obama’s nominee, insisting that the next president should make the choice. But with Donald J. Trump’s poll numbers sliding, the Democratic leadership sees an opening to derail Republicans who are facing re-election by blaming them for the delay.

“We’re not going to back off,” Senator Harry Reid of Nevada, the Democratic minority leader, said in an interview this week. “There will be things we are going to do to draw more attention to this.”

He predicted that the Republican presidential nominee’s prospects would decline further by next month, adding, “By then, they are going to have to look for some way to break from Donald Trump.”