Rep. Seth Moulton (D-Mass.), center, speaking with others during a Texas campaign event for Joseph Kopser in 2018. (Tamir Kalifa/For The Washington Post)

Rep. Seth Moulton of Massachusetts launched a bid Monday for the Democratic nomination for president, becoming the 19th candidate in the most crowded primary in the party’s modern history.

“I’m running because I’m a patriot, because I believe in this country and because I’ve never wanted to sit on the sidelines when it comes to serving us,” Moulton said in an interview on ABC’s “Good Morning America” hours after posting an announcement video on his website that highlighted his military service.

The Massachusetts congressman, 40, a former Marine Corps captain who served in the Iraq War, was first elected in 2014 after ousting a scandal-plagued Democratic incumbent. He is the sixth current or recent member of the House to enter the Democratic contest and one of three veterans seeking the nomination.

Like Rep. Tulsi Gabbard (D-Hawaii) and South Bend, Ind., Mayor Pete Buttigieg, he has touted that experience as a reason he can defeat President Trump.

During his interview on ABC, he called Trump “the most divisive president in American history” and said he would stand apart from others in the Democratic field because he is “going to talk about patriotism, about security, about service.”

“These are issues that for too long Democrats have ceded to Republicans, and we’ve got to stop that,” Moulton said.

Moulton was to head to New Hampshire, the first presidential primary state, on Tuesday.

“We’ve never had a more reckless commander in chief,” Moulton said last month during his listening tour of the first four primary states. “Veterans aren’t going to let themselves be defined by a draft dodger.”

Moulton is also the second House member to mount a presidential bid after waging an effort to replace Nancy Pelosi (D-Calif.) as House speaker. Like Rep. Tim Ryan (D-Ohio), who entered the race earlier this month, Moulton campaigned for multiple House candidates last cycle while calling for fresh leadership. And like Ryan, Moulton eventually voted to give the gavel back to Pelosi, but only after some negotiations over the party’s rules.

“We have a speaker who’s doing a great job with the president,” Moulton told The Washington Post last month. “We have an agreement on term limits that’s going to give this new generation of leaders a voice. Honestly, in some ways, that’s better than if we just had three new leaders in the House.”

While Moulton made some allies around the country in 2018 — his Serve America PAC raised $4.3 million and helped elect 21 candidates — he has not been strongly identified with either moderates or the insurgent left wing.

During his appearance on ABC, he sought to put distance between himself and Sen. Bernie Sanders (Vt.), who identifies as a democratic socialist.

“I’m not a socialist, I’m a Democrat. I want to make that clear,” Moulton said.

As Moulton considered a run, his political team conducted focus groups in New Hampshire and South Carolina that found Democratic voters prizing an electable candidate above all other factors — and not wedded to older, better-known candidates. They were warm, his team said, to the idea of supporting a candidate from a younger generation who could combat the president’s national security record.

Moulton has echoed some of the party’s left-wing activists in calling for the end of the electoral college and suggesting that “court-packing” might be necessary to combat conservative gains in the judicial branch. At the same time, he has criticized the concept of Medicare-for-all, citing his frustrations with the Veterans Affairs health-care system to say that single-payer health care comes with downsides.

“Maybe some of these folks should join me the next time I go in for surgery,” Moulton said of single-payer advocates in a March interview with The Post.

The Massachusetts congressman also separates himself from the two veterans already in the race with his focus on foreign policy. Gabbard, who endorsed Sanders for president in 2016, has called for an end to “wasteful regime-change wars,” while Buttigieg has talked little about foreign policy. Moulton, who held multiple roundtables with veterans as he explored his 2020 bid, has called for modernizing America’s military posture by prioritizing cyber and autonomous weapons instead of large military equipment.

“Russia and China have an inherent advantage over us by being more budget-restrained and less politically constrained,” he said in a February speech at the Brookings Institution. “They don’t have the luxury of trying to compete with our big, expensive legacy weapons systems, so they have to develop the smaller, cheaper next-generation weapons.”

Moulton begins this bid as an underdog, though he often notes he began his 2014 House run in a similar position.

In 2018, he raised $2.4 million to defeat a Republican candidate who put up only token opposition; if he were to switch back from the presidential race to run for another House term, he would have until June 2020 to decide.