With the revelation by prosecutors on Wednesday that a tabloid publisher admitted to paying off a Playboy model, key participants in two hush-money schemes say the transactions were intended to protect Donald J. Trump’s campaign for president.

That leaves Mr. Trump in an increasingly isolated and legally precarious position, according to election law experts. Hundreds of thousands of dollars in payments made in 2016 to keep two women silent about alleged affairs are now firmly framed as illegal campaign contributions.

The news about the publisher, the parent company of The National Enquirer, came on the same day that Mr. Trump’s former lawyer Michael D. Cohen was sentenced to three years in prison in part for his involvement in the payments. “I blame myself for the conduct which has brought me here today,” Mr. Cohen said, “and it was my own weakness and a blind loyalty to this man” — a reference to Mr. Trump — “that led me to choose a path of darkness over light.”

Mr. Cohen said the transactions were an effort to cover up the president’s “dirty deeds,” a claim that was buttressed when federal prosecutors announced that the tabloid publisher, American Media Inc., said it had bought one of the women’s stories to ensure she “did not publicize damaging allegations about the candidate.”