The early departure 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, which caught congressional leaders by surprise, adds to a growing number of Cabinet vacancies in the Trump administration as well as a confirmations backlog in the Senate.

President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE on Sunday announced he is removing Mattis, who resigned on Thursday, two months earlier than his scheduled departure at the end of February.

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Also due to depart at year’s end are Interior Secretary Ryan Zinke Ryan Keith ZinkeTrump extends Florida offshore drilling pause, expands it to Georgia, South Carolina Conspicuous by their absence from the Republican Convention Trump flails as audience dwindles and ratings plummet MORE and U.S. Ambassador to the United Nations Nikki Haley Nimrata (Nikki) Haley'The soul' versus 'law and order' Author Ryan Girdusky: RNC worked best when highlighting 'regular people' as opposed to 'standard Republicans' GOP lobbyists pleasantly surprised by Republican convention MORE.

The Senate will have to hold hearings and vote to confirm their replacements, as well as vote on William Barr, whom Trump has appointed to succeed Jeff Sessions Jefferson (Jeff) Beauregard SessionsGOP set to release controversial Biden report Trump's policies on refugees are as simple as ABCs Ocasio-Cortez, Velázquez call for convention to decide Puerto Rico status MORE as attorney general, and Andrew Wheeler, the acting administrator of the Environmental Protection Agency, whom Trump says he plans to nominate to permanently hold the post.

The president says he will replace Haley with State Department spokeswoman Heather Nauert.

That means the Senate will be cluttered by five high-profile confirmation fights at the beginning of 2019.

A sixth confirmation battle could happen if Trump pushes out Homeland Security Secretary Kirstjen Nielsen Kirstjen Michele NielsenMore than million in DHS contracts awarded to firm of acting secretary's wife: report DHS IG won't investigate after watchdog said Wolf, Cuccinelli appointments violated law Appeals court sides with Trump over drawdown of immigrant protections MORE, who has come under fire from the president.

Senate Republicans are calling on Trump to replace Mattis with a like-minded candidate, even though Mattis announced Thursday that he was stepping down from the top Pentagon post because of differences with the president over national security policy.

“I’d like a Mattis clone. I think we all would,” Senate Homeland Security Committee Chairman Ron Johnson Ronald (Ron) Harold JohnsonThe Hill's 12:30 Report: Ginsburg lies in repose CHC leaders urge Senate to oppose Chad Wolf nomination Top GOP senators say Hunter Biden's work 'cast a shadow' over Obama Ukraine policy MORE (R-Wis.) told reporters earlier this week.

“This is disconcerting to say the least. We lost McMaster, we lost Kelly, we’re losing Mattis,” he added. “These are people that first of all served their country with distinction and they bring a very needed perspective into the administration,” he added, referring to former national security adviser H.R. McMaster, who left the administration in April, and outgoing White House chief of staff John Kelly John Francis KellyMORE.

Mattis said he would leave after Trump surprised lawmakers by announcing a decision to withdraw 2,000 U.S. troops from Syria.

Brett McGurk, the special envoy for the coalition to defeat the Islamic State in Iraq and Syria, also announced his resignation Friday.

While lawmakers aren’t grieving over the departure of Zinke, who has come under an ethical cloud related to a controversial land deal involving his family in Montana, they are uneasy about the departures of Mattis and Kelly, who were highly respected by both sides on Capitol Hill.

CNN host Jake Tapper asked 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.) on Sunday about the departure of several key figures Corker once described as “those people who help separate our country from chaos."

“About three or four or five months ago I noticed a real change,” Corker said. “I think the president has felt, ‘Look, I’ve got this now.’ ”

Corker said the president appears to be putting less value on experts such as Mattis and Kelly, citing the decision to pull out of Syria despite the advice of national security experts.

The Senate still had to act on 195 executive nominees as of a week ago, according to a tally by The Washington Post and the Partnership for Public Service.

In addition, there’s a growing backlog of judicial nominees.

There were 31 judicial nominees awaiting floor action last week, according to the Senate Judiciary Committee.

Senate leaders traditionally confirm a package of judges before wrapping up a Congress but Senate Democratic Leader Charles Schumer Chuck SchumerCruz blocks amended resolution honoring Ginsburg over language about her dying wish Senate Democrats introduce legislation to probe politicization of pandemic response Schumer interrupted during live briefing by heckler: 'Stop lying to the people' MORE (N.Y.) refused to do so this month under heavy pressure from liberal activists.

The Senate will be tied up for the first half of next year over what are likely to be controversial confirmation hearings and votes.

Republicans, however, will have some more room to confirm Trump’s nominees, as they picked up two seats and will control a 53-47 Senate majority in 2019.