Failing to attract any substantial support in polls, Mr. Moulton did not qualify for inclusion in any of the televised debates, which required candidates to meet certain benchmarks in polling and financial support.

“Candidly, getting in the race late was a mistake,” Mr. Moulton said. “It was a bigger handicap than I expected.”

While Mr. Moulton said he would not “cry about the D.N.C. rules being unfair,” he said the party’s debate setup was not “a smart system to choose the best nominee to take on Donald Trump.”

[Jay Inslee finally achieved national recognition — after dropping out of the race.]

He mentioned Gov. Steve Bullock of Montana, a fellow moderate who has also strained to reach the debate stage, as another candidate disadvantaged by the debate restrictions.

“I’ve always said that veering too far left could result in us losing this election, and that Trump will be harder to beat than most people think,” Mr. Moulton said.

He pointed to health care as an issue where some Democrats were at risk of alienating voters with calls to eliminate private insurance. Voters, he said, were “on the side of strengthening Obamacare” rather than implementing a single-payer system.

Asked if that made him a Biden supporter, Mr. Moulton did not exactly say no.

“I’m not going to endorse anyone right away, but the vice president is a mentor and a friend and I think he’d make a great president,” Mr. Moulton said, adding, “Anybody in this race will be better than Donald Trump and I will enthusiastically support whoever the nominee is.”