As former President Donald J. Trump’s criminal trial winds down, a center-left group is trying to goad the Republican presidential candidate into testifying.

“Take the stand, Donald, or admit you’re a coward,” blares a new ad from the group, Third Way, which highlights Mr. Trump’s past comment that “if you’re innocent, why are you taking the Fifth Amendment?”

The five-figure digital ad buy is running “in all the places Trump will be,” said Matt Bennett, a co-founder of Third Way. He said the spot would run “in New York, especially around Trump Tower and the courthouse, Mar-a-Lago, and in Dallas” near the National Rifle Association meeting where Mr. Trump is expected to speak on Saturday.

“He has talked tough for years about how only guilty people and mobsters take the Fifth,” Mr. Bennett said. “Now it appears he’s going to do that himself, and we wanted to remind him of that and see if we could taunt him into testifying, frankly, because it might not go well for him if he did that.”

Mr. Trump’s lawyers told the judge Thursday that he had yet to decide whether he would testify. Mr. Trump has invoked the Fifth Amendment before.

Mr. Trump is charged in this trial with falsifying 34 business records related to the reimbursement of a hush-money payment to a porn star, Stormy Daniels. He has denied the charges.

The ad buy was first reported by Politico.