A vulnerable Senate Republican in North Carolina tries to halt his challenger’s momentum.

Senator Thom Tillis of North Carolina, one of the Republicans’ most vulnerable incumbents, tried to turn around his faltering re-election campaign on Monday night, clashing with Cal Cunningham, his Democratic challenger, in a televised debate that largely focused on the coronavirus pandemic.

The contest in a key swing state is one of a handful of races that could determine control of the Senate next year, and almost every recent public poll shows Mr. Tillis, a first-term incumbent who is mostly allied with President Trump, trailing Mr. Cunningham, a former state senator and Iraq war veteran.

Seeking to halt that momentum, Mr. Tillis repeatedly tried to chip away at Mr. Cunningham’s image as an inoffensive moderate, painting him instead as a craven, ladder-climbing liberal who would “say anything to get elected.”

Mr. Tillis implied Mr. Cunningham would defund the police, a position his rival rejected. He leaned heavily on his own status as an incumbent, highlighting bipartisan work done by Congress to provide relief to millions of Americans suffering from economic hardships caused by the coronavirus.