Establishment Republicans are warming to the idea of Donald Trump Donald John TrumpObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE as the GOP standard-bearer.

Two House Republican committee chairmen, Reps. Bill Shuster (Pa.) and Jeff Miller (Fla.), announced their support for Trump on Thursday. And sources said more rank-and-file Republicans are expected to follow suit, including longtime Rep. John Duncan Jr. (Tenn.).

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Furthermore, regular meetings between House lawmakers and top Trump aides, once sparsely attend, have suddenly become must-see events.

Trump’s convention manager, Paul Manafort, huddled with House Republicans on Thursday in a meeting described by one attendee as “standing room only.”

Among the Republicans in attendance were Pennsylvania Reps. Tim Murphy and Mike Kelly. Neither have endorsed Trump though Kelly said he voted for him in Tuesday’s presidential primary.

Trump supporter Rep. Tom Reed (R-N.Y.) noted the jump in attendance after attending Thursday's meeting at the Capitol Hill Club next to the Republican party headquarters.

“A month ago when we started, there were a lot of empty seats. Today it was packed,” Reed said.

“The realization is that Donald Trump is going to be our nominee,” Reed said. “We’re coming to the end of the process; it’s time to unite the party and take on Hillary Clinton Hillary Diane Rodham ClintonWhat Senate Republicans have said about election-year Supreme Court vacancies Bipartisan praise pours in after Ginsburg's death Trump carries on with rally, unaware of Ginsburg's death MORE.”

Trump is by no means the establishment favorite at this point.

Some conservatives say they’re embarrassed by the thought that their party might put him up as their nominee for president.

And many Republicans believe Trump will be a disastrous general election candidate that will cost the party not just the White House but also majority control in the Senate and potentially the House.

There is a movement of anti-Trump conservatives, led by operatives with ties to influential establishment figures who are hellbent on stopping Trump at any cost.

And the backlash against the front-runner has been strong enough that lawmakers who once despised Ted Cruz Rafael (Ted) Edward CruzSenate Republicans face tough decision on replacing Ginsburg Cruz: Trump should nominate a Supreme Court justice next week Renewed focus on Trump's Supreme Court list after Ginsburg's death MORE — Sen. Lindsey Graham Lindsey Olin GrahamSenate Republicans face tough decision on replacing Ginsburg Democratic senator calls for eliminating filibuster, expanding Supreme Court if GOP fills vacancy What Senate Republicans have said about election-year Supreme Court vacancies MORE is the case study here — have thrown their support behind the Texas Republican, believing he’s the last hope to stop Trump.

A senior adviser to the Never Trump PAC dismissed the latest round of endorsements and scolded the lawmakers who have thrown their support behind Trump.

“Out of over 300 Republicans in Congress, a small handful have now reluctantly agreed to hear from Donald Trump's staff,” the adviser said.

“They should look around at every meeting and imagine how many of their colleagues won't be back next year because Trump destroys our down ballot opportunities, and they should consider the implications of losing a historically large Republican majority and handing Hillary Clinton the White House and a Democratic Congress.”

Still, there’s an undeniable thaw between Trump and lawmakers on Capitol Hill.

The ranks of Trump’s congressional endorsements will swell from nine to 12 on Thursday, and the addition of two House chairmen will give the front-runner added heft as he seeks further inroads with the GOP establishment.

Shuster is the chairman of the House Transportation and Infrastructure Committee, and will give an added boost to Trump as one of 54 unbound delegates from Pennsylvania who can support whomever they choose at July's Republican National Convention in Cleveland.

Trump’s support from House members in Pennsylvania has already paid off.

Two early supporters, Reps. Tom Marino and Lou Barletta Louis (Lou) James BarlettaBottom Line Ex-GOP congressman to lead group to protect Italian products from tariffs Head of Pennsylvania GOP resigns over alleged explicit texts MORE, worked on the ground for him ahead of Tuesday’s primary there and helped him finally beat Cruz at the inside game of getting supporters elected as delegates.

Miller, meanwhile, leads the Veterans Affairs Committee, a group that has been a central focus of Trump’s outreach. The retiring Florida congressman has been informally advising Trump on military and veterans issues.

Those endorsements could beget other endorsements as lawmakers lobby on Trump’s behalf to their colleagues or feel safe to announce their support for a candidate who has been a lightning rod for controversy.

So far, Trump still only has one supporter in the upper chamber, Alabama Sen. Jeff Sessions Jefferson (Jeff) Beauregard SessionsTrump's policies on refugees are as simple as ABCs Ocasio-Cortez, Velázquez call for convention to decide Puerto Rico status White House officials voted by show of hands on 2018 family separations: report MORE (R-Ala), who was on hand to spin for the candidate after his much-anticipated foreign policy speech at The Mayflower Hotel in Washington on Wednesday.

The speech received mixed reviews within conservative circles but was surprisingly praised by Sen. Bob Corker Robert (Bob) Phillips CorkerHas Congress captured Russia policy? Tennessee primary battle turns nasty for Republicans Cheney clashes with Trump MORE (R-Tenn.), the chairman of the Foreign Relations Committee and a well-regarded establishment figure who said he had a “solid” phone call with Trump on Thursday.

“One of the things I like about Trump’s speech was the foreign policy establishment here in Washington has not been spectacular in their prognosis about what we should do,” Corker told reporters. “And I like the fact that he’s challenging the foreign policy establishment.”

Corker isn’t endorsing Trump, but the chairman said he’ll back him if he wins the nomination.

Outspoken Trump opponents downplayed the burst of endorsements on Thursday.

“I think generally speaking, congressional endorsements are not that relevant. It’s not going to change anything. We have to see what happens in Cleveland. That’s the only thing that matters right now,” said Rep. Carlos Curbelo (R-Fla.), who first backed Jeb Bush in the primary and has vowed he won’t vote for Trump if he’s the nominee.

Speaker Paul Ryan Paul Davis RyanKenosha will be a good bellwether in 2020 At indoor rally, Pence says election runs through Wisconsin Juan Williams: Breaking down the debates MORE (R-Wis.), who will be the ceremonial chairman of the Cleveland convention, said members of his caucus are free to do as they choose.

“Our members are free to endorse or not to endorse however they want to,” Ryan said at a news conference Thursday. “Either a candidate will get to 1,237 before Cleveland or the delegates through whatever round of voting will pick the nominee. My job is to make sure it is done smoothly and by the rules, and I’m sticking to that.”

Julian Hattem contributed.