Senate Republicans willing to counter President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE on defense and national security are becoming a rarity on Capitol Hill.

The death of Sen. John McCain John Sidney McCainAnalysis: Biden victory, Democratic sweep would bring biggest boost to economy The Memo: Trump's strengths complicate election picture Mark Kelly: Arizona Senate race winner should be sworn in 'promptly' MORE (R-Ariz.) marks the start of the waning of an already small group of GOP senators willing to act as a counterweight to Trump as the president breaks with decades-long Republican policies.

McCain, despite being diagnosed with an aggressive form of brain cancer in July 2017, remained a vocal critic of U.S. foreign policy and national security under the Trump administration with his outspoken opposition to Gina Haspel's nomination to lead the CIA and his support of tougher sanctions against Russia.

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The former senator didn’t mention Trump in his farewell statement released Monday, but he appeared to take a parting shot at the ideological differences that have decided the two men for years.

"We weaken our greatness when we confuse our patriotism with tribal rivalries that have sown resentment and hatred and violence in all the corners of the globe,” McCain said in a statement released by his Senate office. “We weaken it when we hide behind walls, rather than tear them down, when we doubt the power of our ideals, rather than trust them to be the great force for change they have always been.”

His death raises fresh questions about who among the Senate Republican Conference will be willing to criticize the administration on foreign affairs, particularly as so many GOP senators have been wary of a direct confrontation with Trump.

Asked about who could fill the void, Senate Foreign Relations Committee Chairman Bob Corker Robert (Bob) Phillips CorkerHas Congress captured Russia policy? Tennessee primary battle turns nasty for Republicans Cheney clashes with Trump MORE (R-Tenn.) said, “I don’t know, we’ll see.”

“Maybe I’m just being hopeful but maybe after the midterms things will change,” said Corker, who’s retiring in early January.

Pressed on why he thought that November's midterm elections might bring about change, he added, “I don’t know, but I hope it’s going to happen.”

There are few obvious picks for who could fill the gap being left by McCain.

Sen. Joe Manchin Joseph (Joe) ManchinSenate passes resolution reaffirming commitment to peaceful transition of power Hopes for DC, Puerto Rico statehood rise Manchin defends Supreme Court candidate Barrett: 'It's awful to bring in religion' MORE (D-W.Va.) praised McCain on Monday and signaled that he hoped Sen. Lindsey Graham Lindsey Olin GrahamSteele Dossier sub-source was subject of FBI counterintelligence probe Hillicon Valley: Subpoenas for Facebook, Google and Twitter on the cards | Wray rebuffs mail-in voting conspiracies | Reps. raise mass surveillance concerns Key Democrat opposes GOP Section 230 subpoena for Facebook, Twitter, Google MORE (R-S.C.) and others would step forward. Though Graham was a close friend and ally of McCain, he’s also aligned himself closely with the president on issues like Trump possibly firing Attorney General Jeff Sessions Jefferson (Jeff) Beauregard SessionsRoy Moore sues Alabama over COVID-19 restrictions GOP set to release controversial Biden report Trump's policies on refugees are as simple as ABCs MORE.

“I’m hoping that Lindsey and some other people on the Republican side will step up and meet us in the middle,” Manchin told a West Virginia radio station on Monday.

Sen. Dick Durbin Richard (Dick) Joseph DurbinThe Hill's Morning Report - Sponsored by Facebook - Trump previews SCOTUS nominee as 'totally brilliant' Feinstein 'surprised and taken aback' by suggestion she's not up for Supreme Court fight Grand jury charges no officers in Breonna Taylor death MORE (D-Ill.) told CBS’s “Face the Nation” that there wasn’t one senator who would be an obvious pick to fill the void created by McCain’s absence.

“Each and every one of us have to play that role in his memory,” he said. “I'm not sure there's one person that is going to grab the banner and move forward.”

Corker initially suggested that GOP Sen. Ron Johnson Ronald (Ron) Harold JohnsonThe Hill's Morning Report - Sponsored by Facebook - Trump previews SCOTUS nominee as 'totally brilliant' The Hill's 12:30 Report: Ginsburg lies in repose CHC leaders urge Senate to oppose Chad Wolf nomination MORE (Wis.), the chairman of the Senate Homeland Security and Governmental Affairs Committee, could emerge as a check on Trump’s foreign policy. He later later told reporters that he would try to think of who could fill the space.

Sen. Christopher Coons Christopher (Chris) Andrew CoonsMurkowski: Supreme Court nominee should not be taken up before election Battle lines drawn on precedent in Supreme Court fight Sunday shows - Ruth Bader Ginsburg's death dominates MORE (D-Del.) over the weekend said there were five Republicans who he could potentially see stepping up: Sens. Marco Rubio Marco Antonio RubioGOP lawmakers distance themselves from Trump comments on transfer of power McConnell pushes back on Trump: 'There will be an orderly transition' Graham vows GOP will accept election results after Trump comments MORE (Fla.), Todd Young Todd Christopher YoungSenate GOP eyes early exit Why the US should rely more on strategy, not sanctions Davis: The Hall of Shame for GOP senators who remain silent on Donald Trump MORE (Ind.), Cory Gardner Cory Scott GardnerBillionaire who donated to Trump in 2016 donates to Biden The Hill's Morning Report - Sponsored by Facebook - Trump previews SCOTUS nominee as 'totally brilliant' Cook Political Report shifts Colorado Senate race toward Democrat MORE (Colo.), Ben Sasse Benjamin (Ben) Eric SasseMcEnany says Trump will accept result of 'free and fair election' McConnell pushes back on Trump: 'There will be an orderly transition' Trump says he'll sign order aimed at protecting premature babies in appeal to religious voters MORE (Neb.) and Thom Tillis Thomas (Thom) Roland TillisTillis appears to reinforce question about COVID-19 death toll Billionaire who donated to Trump in 2016 donates to Biden Collins: Winner of presidential election will be sworn in next year MORE (N.C.).

Coons, in an interview with The Washington Post, didn't explain his thinking for each of the five. But Rubio, who ran for president in 2016, has been outspoken about Trump's deal with Chinese telecommunications firm ZTE; Gardner and Young are members of the Senate Foreign Relations Committee; Tillis co-sponsored legislation protecting special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE; and Sasse often sounds off in frank terms against the administration.

Rep. Adam Schiff Adam Bennett SchiffSchiff claims DHS is blocking whistleblower's access to records before testimony GOP lawmakers distance themselves from Trump comments on transfer of power Rubio on peaceful transfer of power: 'We will have a legitimate & fair election' MORE (Calif.), the top Democrat on the House Intelligence Committee, also compared Sasse to McCain, noting that he used a Senate floor speech to knock his GOP colleagues for opening the door to Sessions being fired.

"You had two prominent GOP senators say, well, if he wants to get rid of the A.G., we'll help him get a new one, but let's wait until after the midterms,” Schiff said on CNN. “I was proud to see Ben Sasse take issue with that. That was very John McCain–like. We need people like John McCain now more than ever."

But Sasse has come under criticism from both Republicans and Democrats for being seen as unwilling to push back against the administration beyond a tweet or a statement. Washington Post columnist Jennifer Rubin questioned how many times Sasse would "tweet something provocative and then do absolutely nothing to challenge the administration."

The dearth of Trump critics within the Senate Republican Conference isn’t expected to reverse course.

Corker would be a leading contender if he wasn’t retiring, and Sen. Jeff Flake Jeffrey (Jeff) Lane FlakeHow fast population growth made Arizona a swing state Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Republican former Michigan governor says he's voting for Biden MORE (R-Ariz.), a fellow critic of Trump, is also leaving the Senate in January.

Flake told reporters on Monday that he was concerned it would be harder to find Republicans to push back on Trump’s foreign policy, and he used a Senate floor speech to urge his colleagues to mirror McCain.

“We have lately wasted a lot of words in this town doing and being everything that John McCain was not,” Flake said on the Senate floor. “We would do well to allow this moment to affect us in ways reflected not merely in our words, but in our deeds.”

McCain, Corker and Flake represented the three GOP senators most willing to publicly call out Trump for his rhetoric, especially his warm tone toward Russian President Vladimir Putin and his questioning of decades-old alliances like NATO.

The death of McCain and retirement of Corker, in particular, could represent a seismic shift in how willing congressional Republicans are to act as a check on the Trump White House. McCain served as chairman of the Senate Armed Services Committee.

Sen. James Inhofe (R-Okla.) will now become chairman of the committee, after being acting chairman in McCain’s absence during much of the past year. Meanwhile, Sen. Jim Risch James (Jim) Elroy RischWhy the US should rely more on strategy, not sanctions Davis: The Hall of Shame for GOP senators who remain silent on Donald Trump Senators blast Turkey's move to convert Hagia Sophia back into a mosque MORE (R-Idaho) is expected to succeed Corker atop the Senate Foreign Relations Committee next year.

Both Inhofe and Risch are considered to be more closely aligned with Trump than McCain or Corker, and less likely to use the megaphone their committee perches provide to criticize or attempt to rein in the White House.

Senate Republican Whip John Cornyn John CornynQuinnipiac polls show Trump leading Biden in Texas, deadlocked race in Ohio The Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Liberal super PAC launches ads targeting vulnerable GOP senators over SCOTUS fight MORE (Texas) downplayed the likelihood that there will be a lack of Republicans willing to criticize Trump.

“I think senators feel completely free to disagree with the president on policy matters as they come up,” he said. “I don’t expect that to change.”

But Democrats, as well as some Republicans, are urging their colleagues to try to emulate McCain’s willingness to work across the aisle on controversial issues, even when they bucked his own party.

Sen. Susan Collins Susan Margaret CollinsThe Hill's Campaign Report: Trump faces backlash after not committing to peaceful transition of power Billionaire who donated to Trump in 2016 donates to Biden Credit union group to spend million on Senate, House races MORE (R-Maine), a moderate senator who often breaks with her party on health care, praised McCain as being “very brave” for getting into the middle of controversial policy fights.

“If his death could cause us all to reflect on the way the Senate used to be and try to honor his legacy by not trying to constantly score partisan political points but rather work together for the good of the country, I can think of no finer legacy for John McCain,” she told a Maine radio station.

Senate Minority Leader Charles Schumer Chuck SchumerPelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' 3 reasons why Biden is misreading the politics of court packing Cruz blocks amended resolution honoring Ginsburg over language about her dying wish MORE (D-N.Y.) read part of McCain’s farwell statement from the Senate floor and praised him as being able to transcend partisan battle lines.

“We can honor him by trying to carry out the principles he lived by. We can try, as he did, to put country before party. We can try, as he always did, to speak truth to power,” Schumer said. “And we can try … to restore the Senate to its rightful place in our national political life.”