When a fledgling Republican group released an inflammatory ad in Arkansas last week warning that “white Democrats will be lynching black folk again” if liberals came to power, it was immediately criticized on social media by members of both parties .

“I condemn this outrageous ad in the strongest terms,” said Arkansas Representative French Hill, a Republican. “There’s no place in Arkansas for this nonsense.”

The group, “Black Americans for the President’s Agenda,” had made the ad in support of Mr. Hill’s campaign.

“This radio ad is disgraceful and has no place in our society," said Clarke Tucker, the Democrat running against Mr. Hill in Arkansas’s Second Congressional District.