A GOP lawmaker joked Saturday that 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 already has someone in place who could succeed outgoing 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: White House budget chief Mick Mulvaney Mick MulvaneyOn The Money: House panel pulls Powell into partisan battles | New York considers hiking taxes on the rich | Treasury: Trump's payroll tax deferral won't hurt Social Security Blockchain trade group names Mick Mulvaney to board Mick Mulvaney to start hedge fund MORE.

Mulvaney, a former conservative congressman from South Carolina, has held several roles in the Trump administration over the past two years — some at the same time.

After Trump announced Saturday that Zinke would leave the administration at the end of the year, Rep. Justin Amash Justin AmashInternal Democratic poll shows tight race in contest to replace Amash Centrist Democrats 'strongly considering' discharge petition on GOP PPP bill On The Trail: How Nancy Pelosi could improbably become president MORE (R-Mich.) joked on Twitter that Mulvaney would be Trump's pick to become the new Interior secretary.

It’s going to be Mick Mulvaney. https://t.co/1HTWDKUgAA — Justin Amash (@justinamash) December 15, 2018

Trump first tapped Mulvaney to serve as director of the Office of Management and Budget (OMB), a position he has held since February 2017.

Mulvaney then retained a dual role in the administration by also becoming the acting head of the Consumer Financial Protection Bureau (CFPB) in November 2017.

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On Friday, Trump named Mulvaney as his acting chief of staff, replacing John Kelly John Francis KellyMORE, who is leaving at the end of the month. But the White House said Friday night that Mulvaney would also keep his title as director of the White House budget office.

His deputy at OMB, Russ Vought, will lead day-to-day operations at the agency while Mulvaney focuses on his chief of staff responsibilities, the White House said Friday.

Mulvaney's responsibilities leading the CFPB have ended, however, with the new permanent head of the agency sworn in this week.

“Mick Mulvaney will not resign from the Office Of Management and Budget, but will spend all of his time devoted to his role as the acting Chief Of Staff for the President. Russ Vought will handle day to day operations and run OMB,” White House press secretary Sarah Huckabee Sanders said.

It's unclear how long Mulvaney will serve as acting chief of staff, or whether he will eventually be moved to a permanent position.

“There’s no time limit. He’s the acting chief of staff, which means he’s the chief of staff. He got picked because the president liked him, they get along,” a senior administration official told reporters on Friday.

Trump announced Saturday that Zinke would exit the administration at the end of the month, though it was unclear whether he had been fired or had offered his resignation.