When the government last shut down, in 2013, Mick Mulvaney considered himself part of “the Shutdown Caucus” — a group of conservative House Republicans who held such a hard line that they were willing to let the lights go out.

Now, four years later, Mulvaney is on a collision course with his former comrades, responsible for convincing intransigent House Republicans to make a different kind of choice and pass a new spending bill by April 28 to avert another shutdown.

The former South Carolina congressman — who was elected in the tea party wave of 2010 and took pride in rejecting his own party’s budget proposals, one after another — now serves as President Trump’s budget director, making him the administration’s chief salesman over the next month on spending matters.

Once an outspoken leader of the House Freedom Caucus, Mulvaney now is tasked with bringing along the group with which his boss has plainly lost patience. Frustrated by their obstruction on health care, Trump last week threatened to destroy Freedom Caucus members in the 2018 midterm elections, even as Mulvaney is working with them to forge consensus on an agreement to keep the government funded.

But there are clear limits to Mulvaney’s influence, as this month’s embarrassing collapse of the Republican health-care bill laid bare. Some Freedom Caucus members speak privately of Mulvaney’s “philosophic convulsion,” as one put it, and are quick to note that he no longer speaks with the ideological purity they came to respect in him, but rather as an agent of a president on the hunt for a deal.

(Claritza Jimenez/The Washington Post)

“All of our lives are composed of trade-offs,” said Rep. Mark Sanford (R-S.C.), a Freedom Caucus member. “Each person has a different calibration on where ‘go’ means ‘go’ and where ‘no’ means ‘no.’ I wouldn’t attempt to suggest for another where their own lines ought to be on that balancing act of personal philosophy and assigned roles or jobs, but what I would say is that I wish Mick the absolute best.”

Trump and his other advisers, however, see Mulvaney as their bridge to the Freedom Caucus, believing he still has unique credibility with the conservative hard-liners, however hostile they may be to some of the administration’s priorities.

“If you have to have somebody on your side that understands the complexity of these [bills] and the stakes around a government shutdown, who would you rather have than Mick Mulvaney?” asked Stephen K. Bannon, the chief White House strategist.

Bannon called Mulvaney “the unsung hero of this administration, because he’s doing yeoman’s work on just about every front. He’s a rock star.”

Marc Short, the White House legislative affairs director, said Mulvaney is “anchored in his core philosophy,” but that he has said, “As much as he loves his colleagues in the House, sometimes it’s less about winning the argument than about actually advancing the ball.”

One example of Mulvaney’s dramatically altered role came with Sanford, who told The Post and Courier in Charleston, S.C., that Trump used Mulvaney as an intermediary to threaten to oust Sanford in retaliation for not supporting the health-care bill. Sanford said Mulvaney told him, “The president asked me to look you square in the eyes and to say that he hoped you voted ‘no’ on this bill so he could run [a primary challenger] against you in 2018.”

The episode marked an uncomfortable evolution for a man once allied with Sanford who saw his previous job in Congress as protecting the American taxpayer against runaway spending — even for the military and even if the cuts he championed caused pain for his constituents.

Now as director of the Office of Management and Budget, however, Mulvaney has proposed a large increase in defense spending, which would be offset by steep cuts in social services such as housing, job training, and after-school activities, as well as foreign aid.

Some of these positions have infuriated antipoverty advocates, particularly his statements that federal assistance for low-income students and the elderly is ineffective.

“Rarely will any program be able to fully accomplish its goals because the needs are so great, but if you took those programs away, there would be a huge impact,” said Libba Patterson, a professor at the University of South Carolina School of Law who ran the state’s social services agency for four years.

Fiscal hawks had a different reaction to Mulvaney’s first budget. Rep. Jeff Duncan (R., S.C.), a Freedom Caucus member who served with him in the South Carolina legislature before they were both elected to Congress in 2010, cheered Mulvaney’s moves. “We were all dancing in the street that Mick was chosen to be OMB director,” he said.

Sen. Rand Paul (R-Ky.) praised Mulvaney, a friend, as “a committed conservative.” But, he said, Mulvaney is wearing “a different hat. He is now representing the administration’s policy, so he doesn’t have the same freedom he had as someone who represented the people of South Carolina.”

During the health-care push, Mulvaney was one of the most visible administration officials. He appeared regularly on television news — Trump thinks he is an especially smooth and punchy communicator, aides said — and lobbied lawmakers incessantly, from negotiating sessions on Capitol Hill to a game of bowling in the White House basement.

Duncan said Mulvaney helped persuade him to support the Affordable Care Act replacement bill, even though many Freedom Caucus colleagues were opposed.

“He couldn’t convince everyone,” Duncan said. “But even when he was in Congress and the Freedom Caucus, he couldn’t convince everyone.”

One of Mulvaney’s selling points for the budget director job was his connection to the Freedom Caucus, Trump aides said, and there is some disappointment that he fell short on selling the health-care bill. But advisers said blame for the failure has fallen on many officials, including White House chief of staff Reince Priebus, not just Mulvaney.

“There’s nothing more that he could have possibly done,” White House press secretary Sean Spicer said of Mulvaney. He called the budget director “a very well-steeped, well-regarded workhorse” who has “an instant sense of credibility on Capitol Hill.”

Mulvaney was well-liked in the House, a rare Freedom Caucus member who made friends with House leaders, including Speaker Paul D. Ryan (R-Wis.).

“I think it’s easy for the media to paint him in a corner philosophically, but his friendships obviously go across the entire spectrum of the Republican conference, and I think that’s why he’s such a great asset,” Short said.

Yet in the Senate, Mulvaney barely won confirmation. Sen. John McCain (R-Ariz.) joined all 48 Democrats and independents in opposing his appointment, in part because of Mulvaney’s past opposition to higher defense spending levels.

Before coming to Washington, Mulvaney, 49, was a state lawmaker and also owned and operated a South Carolina franchise of Salsarita’s Fresh Cantina. He first got elected to the House by unseating one of Congress’s long-serving lions — John Spratt, then the House Budget Committee chairman — in a district that Democrats had controlled for more than 100 years.

Spratt said he was surprised Mulvaney had pulled off getting appointed budget director, arguing that he has no “real experience in budget-making.”

“I’m still surprised that he was able to pull down a prize like OMB,” Spratt said. “It’s one of the most difficult jobs in the United States. He’s got to prove himself worthy of the job.”

Mulvaney’s colleagues said he has proven a quick study, and that he helps them see around corners politically.

Mulvaney has instructed the career staff at the budget office to read Trump’s 1987 bestseller, “The Art of the Deal.” He supported Paul in the 2016 presidential primaries, but came around to Trump once he emerged as the presumptive nominee.

Now one of Trump’s employees — he attends the White House senior staff meetings every morning — Mulvaney is forging a bond with the president. Aides said that whenever Trump talks about numbers, he summons Mulvaney if he is not already in the Oval Office.

Trump also invited Mulvaney to join him last weekend at Trump National Golf Club in Virginia, according to one of the president’s advisers.

“He can take the mundane — budget policy is not the sexiest thing in the world, let’s face it — and not only make it interesting, but talk to you about the different angles of it,” said Rick Dearborn, a deputy White House chief of staff. “It’s not just the policy piece of it, but his political insights that make it very interesting. He gives you these ‘aha moments’ of, ‘Oh, yeah, I hadn’t thought about that.’”