Representative Seth Moulton of Massachusetts has dropped out of the presidential race, ending a candidacy that emphasized Mr. Moulton’s centrist politics and military service but gained no traction with Democratic primary voters.
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 announced 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.
[Who’s in? Who’s out? Keep up with the 2020 field with our candidate tracker.]
Mr. Moulton said he would run for re-election to the House, representing a coastal district to the north and east of Boston. Several other Democrats filed to run for his seat while Mr. Moulton was a presidential candidate, and he is likely to face a contested primary.
Mr. Moulton said he would also relaunch his political action committee, Serve America, to promote issues related to veterans and the military. Those issues, he said, were not “getting the attention they deserve” in the presidential race.
With Mr. Moulton’s departure, the sprawling Democratic field shrank to 21 candidates.
He is the fourth Democrat to leave the presidential race this summer, following Gov. Jay Inslee of Washington, former Gov. John Hickenlooper of Colorado and Representative Eric Swalwell of California. Mr. Hickenlooper announced on Thursday that he would run for Senate, while Mr. Inslee and Mr. Swalwell are running for re-election to their current posts.
A combat veteran who served in the Iraq War, Mr. Moulton campaigned on themes of strengthening national defense and promoting public service, and criticizing Mr. Trump for damaging the country’s most vital alliances. In May, he revealed that he struggled with post-traumatic stress disorder after returning from war, and called for new policies to attend to the mental health issues of soldiers and veterans. Stanley McChrystal, the retired general who led American forces in Afghanistan, endorsed Mr. Moulton’s campaign.
But Mr. Moulton entered the race late, in a strategic choice he now concedes was a mistake. He announced his candidacy in late April, days before Mr. Biden became a candidate and overshadowed much of the rest of the Democratic field.
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.”
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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.”
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First elected to the House in 2014, Mr. Moulton made a name for himself as an insurgent in and outside of the chamber. He won his seat by defeating an incumbent Democrat, John F. Tierney, in a primary election, and played a rebellious role in the Democratic caucus as a scathing critic of Speaker Nancy Pelosi, alienating senior lawmakers and influential Democratic women in the process.
His appetite for rebellion stirred speculation that he would consider a 2020 primary challenge to Senator Edward J. Markey, a fellow Democrat. Mr. Moulton opted to run for president instead and said this week that a Senate candidacy was off the table.
Mr. Markey is facing a potential challenge from Representative Joseph Kennedy III.
“I haven’t, honestly, been paying attention to that debate,” Mr. Moulton said of the brewing rivalry, “but I’m the product of a primary. I think primaries are healthy.”
Mr. Moulton, who became a father last fall, said he had no regrets about the campaign except his late entry, and said he hoped his infant daughter would one day see him as having done “everything I could to defeat Donald Trump.”
For now, Mr. Moulton said, “she’ll just see her dad around a lot more.”