Rep. Seth Moulton, D-Mass., arrives for the House Democrats' caucus meeting in the Capitol on Thursday, Nov. 15, 2018.

WASHINGTON — Rep. Seth Moulton, D-Mass., became the latest Democrat to enter the 2020 presidential race Monday, launching a presidential campaign focused on national security.

A 40-year-old Marine combat veteran who earned a Bronze Star in Iraq, Moulton's stature has been rising since he won a seat in Congress in 2014 by defeating a long-time Democratic incumbent in a primary.

Moulton may be best known, however, for his opposition to Speaker Nancy Pelosi. His call for new leadership in internal House elections early this year cost him some support back home in his congressional district north of Boston, which includes Salem and the famous fishing town of Gloucester. He later came around to support Pelosi.

Moulton has been traveling to early primary and caucus states as he prepared for a presidential run. This week, he'll visit all four of them again and participate in a service project in each, something he has long made a part of his campaigns.

Moulton is the 19th candidate to enter the very crowded Democratic field. He's hardly a household name and may struggle to stand out from the eight other male candidates.

But his advisers think the primary contest is still wide open and believe voters want a fresh, younger voice who is willing to break with their own party if need be. And his team argues he can carve a place for himself in the field by focusing on national security, which none of the other candidates have prioritized thus far.

He's used his position in the House to recruit and support veteran candidacies with his PAC, which contributed $4.3 million in last year's midterms.

Unlike most other young veterans in politics, Moulton joined the military before the September 11 terror attacks and led one of the first platoons to enter Baghdad during the initial invasion.

The other veterans in the 2020 field are Pete Buttigieg, the mayor of South Bend, Indiana, and Rep. Tulsi Gabbard, D-Hawaii.

Moulton holds three degrees from Harvard — including an MBA and masters of public policy — and attended the elite Phillips Academy prep school in Andover, Mass.

Ideologically, Moulton is progressive on many issues, coming out for the Green New Deal and abolishing the Electoral College and Senate filibuster before some other candidates. But he's temperamentally considered more moderate.