Weeks after he was named acting White House chief of staff, Mr Mulvaney summoned the labour secretary for a tense January encounter that became known inside the West Wing as "the woodshed meeting."
Mr Mulvaney told Mr Acosta in blunt terms that the White House believed he was dragging his feet on regulation rollbacks desired by business interests and that he was on thin ice as a result, according to advisers and a person close to the White House.
Soon afterward, Mr Acosta proposed a spate of business-friendly rules on overtime pay and other policies.
But it was not enough to save Mr Acosta from Mr Mulvaney's ire - and it helps explain why the former federal prosecutor had such tepid administration support last week as he resigned over his handling of a high-profile sex-crimes case more than a decade ago.
The episode illustrates the growing influence wielded by Mr Mulvaney, a former tea party lawmaker who has built what one senior administration official called "his own fiefdom" focused on pushing conservative policies - while mostly steering clear of the Trump-related pitfalls that tripped up his predecessors by employing a "Let Trump be Trump" ethos.
This account of Mr Mulvaney's rising power is based on interviews with 32 White House aides, current and former administration officials, lawmakers and legislative staffers, some of whom requested anonymity to speak candidly.
Mr Mulvaney and the White House declined to make him available for an interview.
Mr Mulvaney - who is technically on leave from his first administration job as budget director - spends considerably less time with Donald Trump than the two previous chiefs of staff, Reince Priebus and John Kelly. And the president has sometimes kept him out of the loop when making contentious foreign policy decisions, advisers say. At a recent donor retreat in Chicago, Mr Mulvaney told attendees that he does not seek to control the president's tweeting, time or family, one attendee said.
Mr Priebus and Mr Kelly had clashed with the president over his Twitter statements and the influence of his eldest daughter and her husband, who are senior advisers.
Instead, Mr Mulvaney has focused much of his energy on creating a new White House power centre revolving around the long-dormant Domestic Policy Council and encompassing broad swaths of the administration. One White House official described Mr Mulvaney as "building an empire for the right wing."
He has helped install more than a dozen ideologically aligned advisers in the West Wing since his December hiring. Cabinet members are pressed weekly on what regulations they can strip from the books and have been told their performance will be judged on how many they remove. Policy and spending decisions are now made by the White House and dictated to Cabinet agencies, instead of vice versa. When Mr Mulvaney cannot be in the Oval Office for a policy meeting, one of his allies is usually there.
"You have a chief of staff with a professional commitment to ensuring that a real policy agenda gets enacted," said Charmaine Yoest, who served in senior roles in the Trump White House and at Health and Human Services before moving to the Heritage Foundation. "You've got to dig in, chart a path forward and stay committed to it, and we welcome his serious approach to policymaking."
But Mr Mulvaney also faces significant obstacles on Capitol Hill, where he made enemies on both sides of the aisle during his three terms as a bomb-throwing House conservative. Democrats openly disdain him as a saboteur, while many key Republicans distrust his willingness to compromise, particularly on fiscal policy. Some GOP senators freely signal that they would rather deal with any other administration official than him.
Mr Mulvaney spends more time in his office than his predecessors, feeling no need to sit in on all of Mr Trump's meetings. He regularly huddles with Joe Grogan, a hard-liner who now leads the Domestic Policy Council, and Russell Vought, a conservative ally who runs the Office of Management and Budget in Mr Mulvaney's absence.
Advisers say a whiteboard in Mr Mulvaney's office has two items with stars beside them: immigration and healthcare. Immigration, however, is largely left to top White House adviser Stephen Miller and, to a lesser extent, presidential son-in-law Jared Kushner, with dim prospects for significant legislation on Capitol Hill. Passing any kind of healthcare bill before the 2020 election is also unlikely, aides say, while budget cuts sought by Mr Vought have died quickly in Congress.
Mr Mulvaney's biggest successes so far have come in deregulation efforts, where he prods agencies to move faster in case Mr Trump loses or Democrats win the Senate in 2020, advisers say.
Aside from the domestic policy shop, Mr Mulvaney has also tapped allies to fill roles in the White House's legislative affairs operation, the Office of Information and Regulatory Affairs, and his old haunts at the OMB. He regularly suggests ideas to all of them.
"What I am seeing is that Mulvaney cares about the domestic agencies much more than the prior chiefs of staff did," said Tammy McCutchen, a former Labour Department official in the George W Bush administration who is now a partner at the Littler Mendelson law firm. "They're holding the agencies accountable to move forward on regulations."
In the past two months, he has forced out the chiefs of staff at the Department of Health and Human Services, White House aides said, and the Labour Department amid policy disputes with them and their respective secretaries.
Caitlin Oakley, an HHS spokesperson, disputed the White House account. She said Peter Urbanowicz "left on his own accord, and any statements to the contrary are 100 per cent false."
Late on Sunday, Mr Urbanowicz also said he was not forced out, providing evidence from February and March of his intent to leave in June.
Mr Mulvaney and Mr Grogan have repeatedly clashed with HHS secretary Alex Azar, overruling him, for example, on ending the funding of medical research by government scientists using foetal tissue.
Emma Doyle, Mr Mulvaney's deputy, has sought to control all presidential events and the president's schedule - asking officials to submit formal proposals for why they should be in the room and controlling who is usually in the room. She also leads a weekly meeting on presidential events. Ms Doyle was recently in charge of a review of the president's immigration agencies and led a months-long hunt earlier this year for the person responsible for leaks of the president's internal schedules.
"Everything is controlled. The only people not under his thumb are Kudlow and Bolton," said one senior administration official, referring to economic adviser Larry Kudlow and national security adviser John Bolton.
Where former chiefs of staff Reince Priebus and John Kelly were more deferential to Cabinet members, Mr Mulvaney has told them they are being judged on how much they can deregulate, with the policy council monitoring them daily. He is pushing for faster rollbacks of rules enacted by former president Barack Obama before Mr Trump's first term ends, such as restricting what falls under the Clean Water Act and halting implementation of higher fuel-economy standards, according to administration officials.
The president has blessed Mr Mulvaney's operation, White House aides said, and Mr Trump considers his chief of staff an emissary to movement conservatives who have been vital to his presidency. But some Trump advisers say the president has no idea what Mr Mulvaney and his aides do all day.
Mr Mulvaney and Mr Vought, among others, have sought to convince Mr Trump to care more about cutting spending and the deficit. But Mr Trump has rebuffed many of their proposed cuts as deficits soar.
Mr Trump recently told West Wing aides that Senate majority leader Mitch McConnell, R-Ky, told him no politician had ever lost office for spending more money. Two people with direct knowledge confirmed that Mr McConnell delivered that message in a June phone call about budget sequestration.
Although pleasing to businesses, Mr Mulvaney's efforts are also heartening to social conservatives, who say they are finding a more open reception than before.
For instance, a new rule released in May gives healthcare providers, insurers and employers greater latitude to refuse coverage for medical services they say violate their religious or moral beliefs. That policy is facing legal challenges. The same month, the White House proposed a rollback of Obama-era rules that banned discrimination against transgender medical patients. Another rule, also being challenged in the courts, bans taxpayer-funded clinics from making abortion referrals.
"We're just taking the president's challenge seriously to look everywhere and come up with options for deregulation that spurs economic growth," Mr Vought said in an interview. "You have an administration that's in sync and everyone is talking to each other."
Mr Mulvaney - who has acknowledged to other advisers that he knows little about foreign policy - has installed a deputy for national security, Rob Blair, who regularly battles with Mr Bolton and his allies. Mr Mulvaney and Mr Bolton are barely on speaking terms, and Mr Blair has regularly challenged Mr Bolton's subordinates, according to people familiar with the relationship.
Mr Mulvaney has also been a key backer internally of Halil Suleyman Ozerden, whom Mr Trump nominated for the 5th Circuit Court of Appeals last month despite misgivings from conservatives, according to people familiar with the matter. Mr Ozerden and Mr Mulvaney have known each other for years, and Mr Mulvaney was a groomsman in Mr Ozerden's wedding. Mr Mulvaney vouched for him in a private conversation with senator Lindsey Graham, R-SC, who chairs the committee that will take up Mr Ozerden's nomination.
The former House Freedom Caucus member's sway in Congress is limited, however. GOP aides routinely trash Mr Mulvaney in private and say he has done little to improve his image from his House days, when he was a leading antagonist in forcing government shutdowns and other hardball tactics. Mr McConnell has told others on Capitol Hill that he would prefer to deal with treasury secretary Steven Mnuchin.
In a recent interview, Senate Appropriations Committee chairman Richard Shelby, R-Ala, paused for 10 seconds when asked whether Mr Mulvaney was a productive force, particularly during a meeting with key principals in the office of House speaker Nancy Pelosi, D-Calif, in June.
Mr Shelby eventually said Mr Mulvaney was "engaged," then the senator pointed out Mr Mnuchin was the lead negotiator on behalf of the administration in the fiscal talks.
The bad blood between Mr Mulvaney and Democrats is even more obvious.
Senator Jon Tester, D-Mont, recalled being pleasantly surprised when the White House reached out to a half-dozen deal-minded Democratic senators in April, wanting to discuss the influx of migrant children at the border.
But he said there was no follow-up from the White House. Later, Mr Tester saw Mr Mulvaney on television complaining that the administration had met with Democrats to talk about problems on the southern border but that they were not working to address them.
“I think it was about Mulvaney being able to get on national TV and say, 'We met with the Democrats,'” Mr Tester said. “It was apparent to me that that was the political agenda behind it. It wasn't about getting anything done. It was about laying blame.”
Mr Mulvaney appears fully aware of his shortcomings with lawmakers, joking to others in the White House about his unpopularity on Capitol Hill. "I know they'd rather deal with Mnuchin," Mr Mulvaney has said, according to two White House officials.
Senator Kevin Cramer, R-ND, who served in the House with Mr Mulvaney, praised his performance but noted that senators are also able to talk to the president directly about concerns.
"He's not there to be a clerk. He's there to lead," Mr Cramer said. "But I think it's also clear that when the president says this is the position, that Mick's more than capable of carrying out the president's position. And I suspect in some cases they're far apart - but in most cases they're pretty well in line."
Mr Mulvaney's relationship with Mr Trump has had its rocky moments. During a recent ABC News interview, the president berated Mr Mulvaney on camera for coughing.
But the two men are unlikely to part ways, advisers say, partially because Mr Mulvaney knows when to leave the president alone - and is a good golfer.
"He takes the phrase chief of staff in the literal way," said Jonathan Slemrod, who led congressional outreach for Mr Mulvaney at the OMB until November. "He's the chief of the staff. He's not chief of the president. He thinks Trump is a political genius and doesn't second-guess a lot of his decisions."
Mr Mulvaney has joked about being an acting chief of staff, arguing that there is no practical difference.
"You could make me the permanent chief of staff tomorrow and he could fire me on Thursday," Mr Mulvaney said of Mr Trump at an 11 June fiscal summit sponsored by the Peterson Foundation. "Or you could leave me as the acting chief of staff and I could stay to the second term. It doesn't make any difference."
He added, "I'll stay as long as I feel like he values my opinion and I like working for him, and both those things are happening right now."
The Washington Post
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