Democrat Rep. Seth Moulton to run for president in 2020: 'I'm not a socialist. I'm a Democrat.'

Rep. Seth Moulton of Massachusetts announced he's mounting a bid for president in 2020, expanding the Democratic field to 19 candidates.

"I'm here to tell you, and to tell America, that I'm running for president of the United States," Moulton told ABC News Chief Anchor George Stephanopoulos on "Good Morning America" Monday.

Moulton, 40, a Marine veteran and three-term congressman from Massachusetts, is one of many Democrats from Capitol Hill to join the race, including potential front-runners like Sens. Bernie Sanders of Vermont, Elizabeth Warren of Massachusetts, Cory Booker of New Jersey and Kamala Harris of California -- not to mention other Democrats who don't currently hold office in Washington but are making a splash, like Mayor Pete Buttigieg of South Bend, Indiana, and former Texas Rep. Beto O'Rourke.

On Monday, he sought to set himself apart from the ever-growing pack of Democratic candidates, many of whom have more Washington experience and star power, saying on "GMA," "I'm not a socialist. I'm a Democrat. And I want to make that clear. Maybe that's a differentiator in this race."

He used health care as an example.

"I think I'm the only candidate who actually gets single-payer health care," he said, adding that he's on a single payer plan himself through the Department of Veterans Affairs. "And I'll tell you, it's not perfect. So if I'm elected, I'm not going to force you off your private health care plan."

He stands apart from many of his primary opponents, including Harris, Warren, Sen. Kirsten Gillibrand and Booker, who support Sanders' Medicare for All bill that would eventually replace the Affordable Care Act with a single-payer system.

Moulton, a former Marine who served in Iraq, is known as an outspoken critic of his own party. He was elected to the House in 2013.

"I returned home and decided to run for Congress because I realized that a lot of Americans are feeling betrayed by Washington the same way that we did in Iraq. Ask anyone who's lost their job to a changing economy, or a child to opioids, or has to choose between heat and food in the winter, they're feeling forgotten," Moulton said in his announcement video posted on Twitter Monday morning.

Moulton has said he is also friends with Buttigieg whom he defended in a recent Twitter post.

Moulton is also the third candidate who represents Massachusetts, joining fellow Democrat and 2020 candidate Warren and former Gov. Bill Weld, the first Republican to challenge President Donald Trump ahead of the primaries.

Moulton pushed back on Warren, who on Friday called for the House to open impeachment proceedings against the president. Though Moulton called for the House to start debates around impeachment last year and said he does believe the president obstructed justice, he also said the Mueller report didn't deliver the "big smoking gun" for the House to move to a vote.

"I don't think it's a right time to have a vote on impeachment until we get the evidence out there. But we should be having this debate, and frankly, we should have been having it starting last year," Moulton said on "GMA."

"I think our party made a mistake by waiting for a smoking gun in the Mueller report and there's not a big smoking gun like we were hoping for. There is a smoking gun about our national security. I mean, we all should be asking, Democrat, Republican alike, why Putin wanted Trump elected president," he said.

Unlike Democrats who will seek to keep their message separate from Trump, Moulton has talked openly about the challenge the president presents to Democrats and finished off his announcement video with a declaration: "I'm running because we have to beat Donald Trump, and I want us to beat Donald Trump because I love this country."

But he knows it won't be easy.

"I'm going to tell [primary voters] that beating Trump is going to be harder than they'd like to think," Moulton said.

Moulton plans to focus his campaign on patriotism, security and service, he said, which are the points where Trump is "weakest."

"These are issues that for too long Democrats have ceded to Republicans and we've got to stop that because this is actually where Donald Trump is weakest," Moulton said. "We've got to take them on on these issues, on these core values that make us strong, and that's what I'm going to do in this campaign. This is going to be a campaign anchored in service, in security and in patriotism."

Moulton is no stranger from straying from party leadership. In 2018, he was part of the small minority of Democrats who voted against Nancy Pelosi returning to the speaker's chair.

At the time, he explained his decision with an echo of his original campaign pitch from when he first ran for Congress in 2013: Washington needs new leadership.

"The American people sent a very clear message in the election last week, that they want new approaches to politics and new leaders in Washington," Moulton said in an interview MSNBC just after the midterms. "If we answer that call for new leadership by reinstalling the same status quo, establishment leadership that we've had in this party since 2006, then we're letting down the American people."

Pelosi eventually secured his vote after she pledged to limit her speakership to two terms, but some saw his objection as a sign of ageism or sexism.

It was not the first time he bucked party leadership -- it's also how Moulton started his career.

When Moulton first entered the race to represent his Massachusetts district, he challenged an 18-year incumbent Democrat. Immediately, Moulton said, he received sharp criticism from the Democratic Party, which warned him about running against a longstanding elected official.

He continued to run, a bold move he said he owes to the training he received as a Marine.

"They said you're going to lose, and by the way, you'll never run for anything again because you don't challenge the establishment," Moulton said in an appearance on "Late Night with Seth Meyers" in 2018.

"Fundamentally, what they were saying to me as a veteran is, 'Do not participate in the democracy you risked your life to defend.' And that's wrong," Moulton said.

The former Marine routinely expresses support for abortion rights and same-sex marriage. During his time in Congress, he's also been a loud advocate for banning semi-automatic assault weapons.

"There's simply no reason for a civilian to own a military-style assault weapon," Moulton wrote in an editorial for the New York Daily News in 2016. "It's no different than why we outlaw civilian ownership of rockets and landmines." Moulton enters the race with a moderate voting record, currently representing the heavily blue district north of Boston, and was most recently ranked as the 65th-most-bipartisan member of the U.S. House of Representatives by The Lugar Center and Georgetown University's McCourt School of Public Policy.

Through his Super PAC, Moulton endorsed veteran candidates in 28 states ahead of the midterm election that saw 21 seats flipped.

Editor's Note: This story has updated to clarify that there are a number of Democratic presidential candidates under 45.