News of Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE's departure from the Trump administration was met with bipartisan shock and concern on Thursday.

Mattis announced he would resign at the end of February, writing in his resignation letter to President Trump that he deserved a secretary "whose views are better aligned with yours."

Sen. Amy Klobuchar Amy KlobucharSocial media platforms put muscle into National Voter Registration Day Battle lines drawn on precedent in Supreme Court fight Sunday shows - Ruth Bader Ginsburg's death dominates MORE (D-Minn.) tweeted that she has "a lot of respect for General Mattis and after what happened yesterday the resignation is sadly understandable," referring to Trump's decision to withdraw U.S. troops from Syria.

I have a lot of respect for General Mattis and after what happened yesterday the resignation is sadly understandable: Jim Mattis Resigns, Rebuking Trump’s Worldview https://t.co/Jkh9wpnp8C — Amy Klobuchar (@amyklobuchar) December 20, 2018

House Minority Leader Nancy Pelosi Nancy PelosiHoyer: House should vote on COVID-19 aid — with or without a bipartisan deal Ruth Bader Ginsburg lies in repose at Supreme Court McCarthy threatens motion to oust Pelosi if she moves forward with impeachment MORE (D-Calif.) told reporters: "I’m sad. I’m shaken by it. I had so much respect for him,” according to CNN's Manu Raju.

Nancy Pelosi reacts to Mattis resignation: “I’m sad. I’m shaken by it. I had so much respect for him,” she told us in the Capitol. She said that she read his “beautiful letter” — Manu Raju (@mkraju) December 20, 2018

Sen. Mark Warner Mark Robert WarnerDemocrats call for declassifying election threats after briefing by Trump officials It's time to upgrade benefits Intelligence chief says Congress will get some in-person election security briefings MORE (D-Va.), vice chairman of the Senate Intelligence Committee, suggested the Trump administration would be destabilized by the Defense secretary's departure.

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"This is scary. Secretary Mattis has been an island of stability amidst the chaos of the Trump administration," he wrote on Twitter. "As we’ve seen with the President’s haphazard approach to Syria, our national defense is too important to be subjected to the President’s erratic whims."

This is scary. Secretary Mattis has been an island of stability amidst the chaos of the Trump administration. As we’ve seen with the President’s haphazard approach to Syria, our national defense is too important to be subjected to the President’s erratic whims. https://t.co/9qPluSGLUc — Mark Warner (@MarkWarner) December 20, 2018

Rep. David Cicilline David Nicola CicillineClark rolls out endorsements in assistant Speaker race Races heat up for House leadership posts The folly of Cicilline's 'Glass-Steagall for Tech' MORE (D-R.I.) called the resignation a “dire omen.”



“James Mattis was the last adult left in the room,” he tweeted. “He was maybe the last person who could tell this President 'No.' His departure is a dire omen for the future of our alliances and the security of our country.”

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Mattis, a former general, was confirmed for the top Pentagon post in a 98-1 vote in the Senate; the sole dissenter was Sen. Kirsten Gillibrand (D-N.Y.).

Sen. Chris Murphy Christopher (Chris) Scott MurphyDemocratic senator calls for 'more flexible' medical supply chain to counter pandemics The Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon GOP chairman to release interim report on Biden probe 'in about a week' MORE (D-Conn.) called Thursday's news “devastating.”

“Just imagine the morale crisis at the Department of Defense right now, having sustained the 1-2 punch of the Syria reversal and Mattis resignation,” he tweeted. “Devastating.”

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High-profile Republicans also expressed concern over the resignation.

Sen. Lindsey Graham Lindsey Olin GrahamSenate GOP aims to confirm Trump court pick by Oct. 29: report The Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Senate GOP sees early Supreme Court vote as political booster shot MORE (R-S.C.) said the news filled him with "great sadness."

"It is with great sadness that I was informed of the resignation of General Mattis," he wrote. "He is one of the great military leaders in American history. He should be proud of the service he has rendered to President @realDonaldTrump and our nation."

It is with great sadness that I was informed of the resignation of General Mattis.



He is one of the great military leaders in American history.



He should be proud of the service he has rendered to President @realDonaldTrump and our nation. — Lindsey Graham (@LindseyGrahamSC) December 20, 2018

Sen. Ben Sasse Benjamin (Ben) Eric SasseTrump says he'll sign order aimed at protecting premature babies in appeal to religious voters Government watchdog recommends creation of White House cyber director position Chamber of Commerce endorses McSally for reelection MORE (R-Neb.) said Mattis was important for curbing Trump’s isolationist tendencies.

“This is a sad day,” Sasse tweeted. "General Mattis was giving advice POTUS needs to hear. Mattis rightly believes that Russia & China are adversaries, and that we are at war with jihadists across the globe who plot to kill Americans. Isolationism is a weak strategy that will harm Americans...”

Sen. Marco Rubio Marco Antonio RubioOvernight Defense: Pentagon redirects pandemic funding to defense contractors | US planning for full Afghanistan withdrawal by May | Anti-Trump GOP group puts ads in military papers Democrats step up hardball tactics as Supreme Court fight heats up Press: Notorious RBG vs Notorious GOP MORE (R-Fla.) said Mattis's resignation letter made it clear that America is headed for a "series of grave policy errors which will endanger our nation, damage our alliances & empower our adversaries."

"I hope we who have supported this administrations initiatives over the last two years can persuade the President to choose a different direction," Rubio added. "But we must also fulfill our constitutional duty to conduct oversight over the policies of the executive branch."