Rep. Seth Moulton (D-Mass.) has received criticism over his opposition to House Minority Leader Nancy Pelosi (D-Calif.) becoming speaker of the House. (J. Scott Applewhite/AP)

Democratic Rep. Seth Moulton, the hard-charging Marine who served four tours of duty in Iraq, is leading the challenge to Rep. Nancy Pelosi and her bid for speaker — and is suddenly taking incoming political fire from all quarters.

Prominent liberal activists are openly floating a primary challenge to the two-term Massachusetts congressman, the instigator of the nearly 20 Democratic foes to Pelosi and one who helped recruit several of the incoming freshmen opposing her.

Outraged by the treatment of the first female House speaker, Pelosi’s backers have characterized the effort as a sexist putsch and urged calls to Moulton’s office as well as the other male Democratic opponents.

Inside the House, Moulton’s ambition and sharp elbows have long been a subject of his colleagues’ grumbling, and his latest fusillade at Pelosi — this time, after a sweeping electoral win — has angered even some of his admirers.

“Is he rubbing people the wrong way with how he’s perceived in this? I would say, yeah,” Rep. Gerald E. Connolly (D-Va.) said.

“Let me get this straight — 17 or 18 who aren’t happy can block the will of over 200 members of the caucus? Where does that end?” he added. “With his zeal for revolution, there’s a lot of wounded left on the battlefield.”

In the aftermath of the Democratic victory, Moulton is sparking intraparty warfare — and infuriating liberals and women around the country — by trying to block Pelosi from reclaiming the gavel in January.

Moulton, 40, and the dozen-plus incumbents and newly elected members insist the House needs new leadership, not the 78-year-old California congresswoman who engineered a Democratic takeover of the House, flipping 36 seats and counting.

“If that many seats change hands, that’s just all the more reason the American people are calling out for change,” Moulton said in a recent interview. “So for our party to respond by saying, ‘. . . We’re going to reinstall the exact same status quo leadership that you voted out many years ago.’ It’s just shooting ourselves in the foot. It guarantees a two-year majority.”

Moulton also offers a more personal reason for opposing Pelosi: the military leader’s ethos of accountability. After losing four-straight election cycles, he says, she shouldn’t see her slate wiped clean after finally helping to win a majority back.

“Never took responsibility for anything, and that’s not what leaders do,” he said. “A good selfless leader shares the credit for a win and owns mistakes.”

Pelosi has dismissed Moulton’s challenge and insists she has the votes.

“I will be speaker of the House no matter what he says,” she said Wednesday.

“She’s wrong,” Moulton fired back Thursday. “We have the votes.”

Moulton’s determination is rooted in the Marine Corps, where he rose from a stint leading a combat infantry platoon in Iraq to becoming a top aide to Gen. David Petraeus.

After leaving the Marines and graduating from Harvard Business School, Moulton vaulted into politics by challenging nine-term Democratic Rep. John Tierney who represented his childhood home on Massachusetts’s North Shore.

Tierney enjoyed close ties to Pelosi — her daughter served as his top aide for a time — and won her endorsement and fundraising help. But Moulton scored a 10-point victory in a hard-fought primary — and with it, a measure of independence from the party leadership.

Since being elected, Moulton has established a reliably liberal voting record with an occasional maverick streak.

Winning a coveted seat on the Armed Services Committee, he emerged as a vocal critic of the Obama administration’s strategy in Iraq and to combat the Islamic State. He has aligned with Republicans on some policy bills, ranging from a ban on the gun accessories used in last year’s mass shooting in Las Vegas to a recent legislation allowing veterans to use medical marijuana.

Moulton has earned an outsize measure of credibility by giving early backing and financial support to a cadre of young insurgents who ran successful campaigns in red-tinted districts across the country.

Thanks to a network of donors rooted in the financial centers of Boston and New York, Moulton’s Serve America PAC and related political committees raised a combined $8 million for the election cycle, putting him in the top echelon of Democratic fundraisers in the House.

Starting with Rep. Conor Lamb (D-Pa.), whose narrow special-election victory in a heavily Republican district last March heralded the coming Democratic wave, Moulton provided crucial early support to a network of fellow veterans and public-service alumni — 10 of whom will join Congress in January.

Six have made clear that they will not support Pelosi as speaker — Reps.-elect Jason Crow (Colo.), Jared Golden (Maine), Max Rose (N.Y.), Mikie Sherrill (N.J.), Elissa Slotkin (Mich.) and Abigail Spanberger (Va.) — with the others undecided.

The group includes several of the breakout stars of the freshman class — such as Sherrill, a former Navy helicopter pilot; Spanberger, a former CIA clandestine agent; and Crow, a former Army officer. With Moulton serving as a mentor, the group has coalesced though a long-running Slack chat into an informal caucus of sorts.

“We’re trying to stay as close as possible,” Crow said Thursday. “We share values, we share that background experience. A lot of us campaigned and talked about very similar approaches to getting things done here, so we’re certainly going to stick close together.”

That cohesion does not necessarily extend to the question of the House leadership; Moulton said there is no litmus test on Pelosi to win his support, and four of the freshmen said in interviews that Moulton was not a factor in their decisions.

But the support network, Moulton said, can help give Pelosi’s freshman opponents the wherewithal to stand their ground amid intense pressure to back down.

“These are all independent people who aren’t afraid to do the right thing if it means going against the party,” Moulton said.

Speculation abounds that Moulton has his eye on everything from the Senate seat now occupied by Democratic Sen. Edward J. Markey to the Massachusetts governor’s mansion to the presidency. He denies having any immediate ambition to do anything but force a leadership shake-up.

Pelosi aides and allies see Moulton as a disingenuous egotist more interested in self-promotion than fulfilling his party’s agenda — a perception that was underscored last year when the Boston Globe published a long, effusive thank-you note to Pelosi that he sent in 2016, barely two months before Moulton unsuccessfully moved to oust her.

Rep. Katherine M. Clark (D-Mass.), who represents a neighboring district, declined to comment on Moulton but said the quest to oust Pelosi put Democrats, “especially our newly elected members . . . in an extremely difficult position.”

Off the Hill, criticism of Moulton among his party’s liberal base is increasingly withering.

Markos Moulitsas, the influential founder of the DailyKos blog, mused early Friday about defeating Moulton in a primary. Other prominent liberal activists have slammed him online as “traitorous” and as one of the “white male ringleaders of this divisive [cow manure].”

“#MA06 will be fun in 2020. So who wants to step up? The money will be there. @sethmoulton and the #FiveWhiteGuys are done,” Moulitsas tweeted.

A spokesman for Moulton acknowledged critical phone calls and messages to his office have spiked, but said so have supportive ones.

One Moulton argument against Pelosi is purely brass-tacks politics: Pelosi remains a drag on Democratic candidates running in Republican-leaning seats. Pelosi backers point to the election results.

“There were so many people who said, ‘Oh my god, if Nancy Pelosi is the leader, we could never win the House of Representatives!’ My friends, the proof is in the pudding,” said Rep. Rosa L. DeLauro (D-Conn).

The endgame Moulton & Co. have in mind remains unclear. They still have not been able to recruit an alternative to run against Pelosi in the Nov. 28 caucus elections, though Rep. Marcia L. Fudge (D-Ohio), who is close to Moulton, is pondering a run.

In any case, Moulton says, the showdown is destined to last until lawmakers cast their speaker votes on the House floor on Jan. 3 — the moment when his group will have leverage to push Pelosi aside. He predicted the freshmen who received his support and who are opposing Pelosi would ultimately stand firm.

“God knows she’s a good arm-twister,” he said of Pelosi, “but it’s clearly not the right thing to do for the party or for the country or for their own reelection prospects.”