Mr. Moulton, 40, said in an interview that he had no immediate plans to endorse another candidate, but he warmly praised former Vice President Joseph R. Biden Jr. Mr. Moulton planned to announce the end of his campaign in a formal speech before the Democratic National Committee on Friday.

Mr. Moulton suggested that most of the other Democratic candidates were also laboring in vain at this point, with only a tiny few — Mr. Biden and Senators Elizabeth Warren and Bernie Sanders — remaining as real competitors for the nomination. He warned in the interview that if Democrats were to embrace an overly liberal platform, it could make it harder for the party to defeat President Trump.

“I think it’s evident that this is now a three-way race between Biden, Warren and Sanders, and really it’s a debate about how far left the party should go,” Mr. Moulton said.