What began earlier this year as a long-shot movement by some Democrats to impeach President Trump hit a new milestone Thursday, with nearly half of the House Democratic majority publicly endorsing a formal impeachment inquiry.

The growing sentiment in Congress has been amplified by calls for impeachment from most of the party’s presidential candidates. One of those candidates, former housing and urban development secretary Julián Castro, suggested at Wednesday’s debate that Trump will taunt Democrats throughout 2020 if they fail to act on impeachment.

The push in the House to oust Trump has been accelerated by testimony from former special counsel Robert S. Mueller III confirming that the president could be charged with obstruction of justice after he leaves office — prompting more than 20 Democrats to announce support for an inquiry. Those calls came amid mounting pressure from liberal activists — applied in some cases by Democratic primary challengers who argue that incumbents, including four powerful committee chairmen, have been too reticent in taking on Trump.

While House Speaker Nancy Pelosi (D-Calif.) continues to stress investigations over impeachment, last week she gave a green light for lawmakers to chart their own course while telling reporters that it would not necessarily change her views.

“I’m willing to take whatever heat there is,” she said.

Interviews with eight of the lawmakers who have publicly backed an impeachment inquiry over the past week showed they are eager to speak out against Trump but loath to breathe hellfire on the party leadership — suggesting the surge of support may not have much immediate impact on Pelosi’s thinking.

Instead, the Democrats have largely cast their announcements as intensely personal decisions that were the product of careful deliberation and close review of Mueller’s report and testimony. Many said they did not want their decisions to be interpreted as criticism of Pelosi or Judiciary Committee Chairman Jerrold Nadler (D-N.Y.), whose panel would lead an impeachment, and none called for an interruption of the six-week recess to return to Washington to launch a formal probe.

Some, in fact, delivered praise for Pelosi while breaking with her more-deliberate approach.

“I think she is phenomenal — you can put that in the paper,” said Rep. Lisa Blunt Rochester (D-Del.), who said in her statement last week that Congress “must stand up and demand accountability.”

“One of the things that she said is that, for each of us, we have to do what we need to do,” she added. “And I respect her for having that respect for us.”

Pelosi refused to answer questions about impeachment during an appearance on Capitol Hill on Thursday.

The speaker’s reluctance about impeachment is based in part on public opinion. A Washington Post-ABC News poll released last month showed 59 percent of Americans believe the House should not begin impeachment proceedings against Trump, while 37 percent believe it should — including 61 percent of Democrats. As of Thursday, 117 of 235 Democrats support an inquiry, according to a Washington Post tally.

But things could change, especially if the campaign waged by liberals gathers steam during the recess scheduled to continue until Sept. 9.

Among those newly backing an impeachment inquiry are two prominent House committee chairmen from New York, Rep. ­Eliot L. Engel of the Foreign Affairs Committee and Rep. Nita M. Lowey of the Appropriations Committee. Both face energetic Democratic opponents in next year’s elections.

Engel said Trump’s “repeated abuses have brought American democracy to a perilous crossroads. Following the guidance of the Constitution — which I have sworn to uphold — is the only way to achieve justice.”

Jamaal Bowman, an educator running against Engel with the backing of the left-wing Justice Democrats, said in an interview that he was “ecstatic” when he heard Engel, a 30-year House veteran, backed an impeachment inquiry.

“We weren’t the only pressure, but we were part of the pressure,” Bowman said, adding: “A statement is not enough. He has to continue to push the conversation to Nancy Pelosi, Jerry Nadler and others to make sure that the impeachment process begins in earnest. We are late on this.”

In response, Arnold Linhardt, a consultant to Engel’s campaign, said the congressman made his decision only after “careful deliberation” and consideration of Mueller’s testimony. “For Mr. Engel’s opponents or anyone to try to politicize a decision of this magnitude is an affront to the American people,” he said.

A coalition of liberal activist groups — including Indivisible, a grass-roots network that helped oust Republicans in dozens of House districts last year, and Need to Impeach, a group funded by billionaire and presidential candidate Tom Steyer — launched the Impeachment August campaign Thursday aimed at helping activists confront lawmakers of both parties about impeachment at public events this month.

David Sievers, who is leading a pro-impeachment campaign for MoveOn.org, called the majority threshold a “major milestone” for House Democrats — one, he speculated, was reached in part because of the looming summer recess.

“I would not want to be in the minority of my party now as a holdout not supporting impeachment,” he said. “I think that members who were on the fence about this and were about to go home and spend time every day with their constituents probably didn’t want to have to explain why the documented criminality of the Trump administration was not enough to warrant an inquiry.”

Even before House members left Washington, Sievers said, MoveOn members made about 7,600 phone calls to congressional offices urging impeachment in the immediate aftermath of Mueller’s testimony. Another 1,000 thank-you calls, he said, went to lawmakers who announced support for an impeachment inquiry after Mueller testified.

The eight Democrats interviewed said constituent activism played only a minor role, or no role at all, in persuading them to support impeachment proceedings. But several said they have gotten significant encouragement after making their announcements.

“The overwhelming response has been, ‘You go, girl,’ ” said Rep. Dina Titus (D-Nev.), who said she was compelled to speak out after Mueller made clear at the hearing that Trump “really didn’t tell the truth.”

Several Democratic freshmen who unseated Republicans last year are among those who have come out in favor of an inquiry in recent weeks. Those lawmakers have given a special boost to impeachment supporters who argue that they rebut the notion that lawmakers in vulnerable districts need to be cautious.

Rep. Jason Crow (D-Colo.), who flipped a suburban Denver seat last year, said Mueller’s testimony was the capstone on a personal “evolution” on the issue that played out over the course of months.

“My district is not one that generally supports Donald Trump — we’re a district that he lost by almost 10 points,” he said. “I talked a lot about accountability and holding the administration accountable, and this certainly is consistent with my promises to do that.”

Another surprising announcement came Monday from Rep. Emanuel Cleaver II (D-Mo.), who for months had publicly urged caution on pursuing impeachment before concluding that more aggressive action was warranted.

“When you impeach a president, you’re actually undoing an election,” he added. “For me, that is so serious that it demands thoughtfulness — and perhaps a deeper level of thoughtfulness than I’ve given on any political issue in my lifetime.”

Reps. Suzan DelBene and Denny Heck were among several members of the Washington congressional delegation who announced their support for an impeachment inquiry in near-unison Sunday after weeks of private conversations.

Heck, a member of the House Intelligence Committee, said he had “marinated” in the 448-page, redacted Mueller report for months before last week’s hearing. “But it was another thing entirely . . . to be able to ask him questions, and just as importantly, to hear my colleagues ask him questions,” he said. Heck wrote his statement backing impeachment at his lakefront cabin on Saturday, having been “plunged into a state of real soulful contemplation.”

DelBene also called Mueller’s testimony a clarifying moment, and while she said she did not intend her statement to serve as a rebuke of Pelosi and other party leaders, she said that she expected the growing drumbeat would not be ignored: “We will continue to have members make decisions like I did personally. And, yes, I do think collectively that will have an impact as we discuss how to move forward.”

At least two others have taken another approach that avoids any confrontation with party leaders: simply declaring that the impeachment process is already underway and does not, in fact, require the endorsement of party leaders or the membership at large.

Rep. Ted Deutch (D-Fla.), a Judiciary Committee member, made that case in a South Florida Sun Sentinel op-ed published Thursday that cited a recent House court filing, and Rep. John Garamendi (D-Calif.) said much the same in a Wednesday interview.

The Mueller hearing, Garamendi said, completed the special counsel investigation process. “The next step is the impeachment inquiry, which I now understand that the Judiciary Committee is proceeding with.”

Several others said a legal argument promoted by members of the House Judiciary Committee was also persuasive — that entering an impeachment inquiry would hand the House additional leverage in the courts to conduct investigations into Trump’s administration, campaign and businesses.

“I absolutely am convinced that it does give us a leg up legally in terms of trying to get the witnesses we need to come to Congress to testify,” said Rep. Julia Brownley (D-Calif.), who made her announcement hours before Mueller testified. “It is critically important that we have every tool in our toolbox to make that happen.”

There are still dozens of prominent Democrats who are urging caution, including many moderate House lawmakers representing some of the 31 districts that Trump won in 2016. Several said Mueller’s testimony last week, largely devoid of new revelations, ought to have ended any notion of impeachment.

On the presidential debate stage Wednesday night, Sen. Michael F. Bennet (D-Colo.) argued that an impeachment surely would result in an acquittal in the GOP-controlled Senate. “And then President Trump would be running saying that he had been acquitted by the United States Congress,” he said, echoing an argument Pelosi has made.

Castro retorted that Democrats like Bennet have been “spooked by 1998” — the year Republicans impeached President Bill Clinton only to see him acquitted by the Senate with his political standing strengthened.

“If they don’t impeach him . . . he’s going to say, ‘You see? You see? The Democrats didn’t go after me on impeachment, and you know why? Because I didn’t do anything wrong,’ ” he said.

Sievers said the groundswell from rank-and-file Democrats could be hard for Pelosi and other Democratic leaders to ignore. “As this becomes a mainstream thing,” he said, “the more people are going to have to start defending why they’re not for it.”

JM Rieger contributed to this report.