President Donald Trump walks with Treasury Secretary Steven Mnuchin to The White House after signing Executive Orders at the Treasury Building on April 21. (Ricky Carioti/The Washington Post)

Shortly before he was sworn in as treasury secretary, Steven Mnuchin spoke with his predecessor to get some advice.

Pay attention to the debt problems in Puerto Rico, Treasury Secretary Jack Lew warned Mnuchin, and remember that China’s currency issues are more complex than the incoming president, Donald Trump, had suggested during the campaign, according to two people briefed on the exchange who spoke on the condition of anonymity to reveal private discussions.

And in pointed remarks, Lew told Mnuchin to take the debt ceiling seriously — or face a potential financial crisis.

Months later, Mnuchin is hurtling toward his first fiasco, unable to get Congress, let alone his colleagues in the Trump administration, on board with a strategy to raise the federal limit on governmental borrowing.

(House Ways and Means Committee)

His struggles are casting doubt on whether the political neophyte, who made his name on Wall Street, has the stature in Washington to press through a vote on a measure that former treasury secretaries of both parties have said is critical to preserving the nation’s reputation for financial stability.

Unlike other issues facing the Trump administration — such as passing a health-care bill and overhauling the tax code — raising the debt limit comes with a hard deadline of late September, according to Mnuchin. Failure to do so could lead the U.S. government to miss paying its obligations, causing what analysts would consider a historic, market-rattling default on U.S. government debt.

“We’re going to get the debt ceiling right,” Mnuchin said in an interview Monday. “I don’t think there is any question that the debt ceiling will be raised. I don’t think there is anybody who intends to put the government’s ability to pay its bills at risk.”

Sensing there could be resistance on Capitol Hill to raising the debt ceiling quickly, he reviewed past debt-ceiling fights. He also holds a weekly meeting with advisers about the government’s cash balance and debt issues.

One former Treasury official, speaking on the condition of anonymity to discuss sensitive agency deliberations, said officials are now “brushing up on options in the ‘crazy drawer.’ ”

In past administrations, Treasury officials have designed plans to prioritize payments to government bondholders so that if the government runs short on cash it could, in at least a technical sense, avoid defaulting on U.S. debt.

Such a scenario would be very difficult to manage because some bills would either be delayed or not paid — but it could be necessary to prevent an actual default. Still, prioritizing payments this way could lead to a spike in interest rates and a stock market crash, analysts have said.

The coming months promise to test Mnuchin, a former Goldman Sachs banker and Hollywood producer who joined the administration as a Trump loyalist, with no experience in government but plenty of experience by the president’s side, serving as campaign finance chairman.

Trump attended Mnuchin’s wedding in June, and on the wall beside Mnuchin’s desk is a news clipping announcing his appointment, signed by Trump along with — in black Sharpie — “I’m very proud of you.”

Beyond the tax code and the debt limit, Mnuchin’s portfolio includes blocking terrorist financing, easing regulations and conveying Trump’s nationalist economic policy at home and abroad.

Mnuchin earlier this summer told lawmakers to raise the debt ceiling in a clean vote that includes no other budget changes before they leave town.

“My preference is to get it clean,” he said Monday. “My preference is to get it done, and my preference is to get it done sooner rather than later.”

But Mnuchin’s push on the debt ceiling was undermined from the start within the White House by Mick Mulvaney, the director of the Office of Management and Budget. Mulvaney is a former Republican congressman and founding member of the House Freedom Caucus who was brought into the White House, in part, to help influence how conservatives would vote on key issues.

Mulvaney publicly questioned Mnuchin’s call for a clean vote, saying that he would prefer spending cuts or other budget changes as part of any proposal to increase the debt ceiling. Some White House and Treasury officials were incensed to see Mulvaney break ranks, said several people involved in internal deliberations who spoke on the condition of anonymity.

Treasury officials complained to the West Wing that Mnuchin’s credibility was being undermined, and Trump told a gathering of Senate Republicans that they should work with Mnuchin, and no one else, on the debt ceiling.

But Mulvaney had sufficiently muddied the administration’s message. And even though Trump told lawmakers that Mnuchin was his point person on the debt limit, the White House still has not publicly come out in favor of a no-strings-attached vote. Top administration officials have now conveyed to Congress that they will support combining an increase in the debt ceiling with other budget changes, as long as Congress works it out soon.

Asked about his relationship with Mulvaney, Mnuchin said, “Mick and I have a very good relationship. I think the press made that out to be more than it is.”

Rep. Mike Quigley (D-Ill.) pressed Mulvaney during a hearing June 21 to explain the conflicting signals from Mnuchin and Mulvaney.

“These two are working against each other,” Quigley said. “It sends mixed messages.”

He added, “It’s also a dangerous message that you don’t have to fulfill your obligations.”

Mulvaney has tried to downplay the rifts but has suggested that his approach was more politically astute.

“It would be foolish of us to come up with a policy devoid of having talked to the Hill,” Mulvaney said to reporters in June.

Lawmakers and congressional aides who have met with Mnuchin describe an earnestness that they viewed as refreshing but also easily outmaneuvered by experienced political hands.

“He’s certainly in the minority in the administration,” said Rep. Mark Meadows (R-N.C.), chairman of the House Freedom Caucus. “The problem is, yes, you could get a clean debt-ceiling, but it would be 180 Democrats in the House with 40 or 50 Republicans, and that’s not a good way to start.”

Meadows said that he recently attended a meeting of eight of the most conservative Senate and House lawmakers about how to handle the debt ceiling and that not once did they consider the idea of backing Mnuchin’s proposal for a clean debt-ceiling increase.

Mnuchin has struggled to give the public an accurate read of how long the Treasury could pay bills before Congress has to act, alternating — sometimes within a matter of minutes — on whether the true deadline is the beginning or end of September.

A Treasury official later clarified that it had sufficient funds to pay all of the government’s bills through September. The Congressional Budget Office, meanwhile, has projected that Treasury should be able to pay all of its bills through early to mid-October.

Senate Majority Leader Mitch McConnell (R-Ky.) recently said that he would hold the Senate open for two weeks in August to take care of unfinished business — namely the health-care bill. It’s unclear whether they’ll tackle the debt limit or whether the House, where the odds of raising the debt limit are even more remote, will remain open.

One of Mnuchin’s challenges is that he still lacks the Washington alliances many Treasury chiefs enjoy.

He has stayed in close contact with friends and former colleagues from the world of finance, such as Blackstone chief executive Stephen Schwarzman and Brian Brooks, who was his vice chairman at OneWest Bank, which Mnuchin ran after acquiring IndyMac’s assets during the financial crisis in 2008. He has also reached out to former treasury secretary Henry M. Paulson Jr. and recently met with other former secretaries, including Lawrence H. Summers and Robert Rubin. And he has discussed the debt ceiling and other issues with Glenn Hubbard, a top Bush administration economic adviser, who came away impressed.

“He has an unassuming manner, but he should not be underestimated,” Hubbard said.

But although Brooks has been nominated as deputy treasury secretary, that role and many other senior Treasury posts remain unfilled. And many Washington conservatives who have spent years backstopping Republican cabinet members know little of Mnuchin’s goals or tactics.

“The guy is literally a name to me and a cipher beyond that,” said Douglas Holtz-Eakin, a longtime Washington GOP economic adviser and former director of the Congressional Budget Office.

Although Mnuchin may be struggling to learn the ways of Washington, he does have an important patron: Trump.

In a way, they share the same pedigree. Both were born into wealth (Mnuchin drove a Porsche in college) and generated even more during their careers.

Trump’s background was in real estate and Mnuchin’s was in banking, but both had a hankering for entertainment and celebrity that drew them close. Even while on Trump’s campaign, Mnuchin remained an active Hollywood producer.

During the campaign, the two traveled together extensively, and Mnuchin surprised a number of Trump’s other aides when he took a front-and-center policy role during the transition into the White House, helping design tax and infrastructure programs that were to be the centerpiece of Trump’s presidency.

People who have met with him at Treasury describe him as polite and curious, with an unabashed affection for Trump that can cloud his message.

During a speech early in his term, Mnuchin said that Trump had “superhuman” health. At a news conference in Canada, Mnuchin criticized former FBI director James B. Comey for leaking details of conversations with Trump. Typically, Treasury chiefs avoid getting dragged into news-of-the-day politics at all costs. And Mnuchin recently said the president “handled it brilliantly” when meeting with Russian President Vladi­mir Putin.

To be sure, Mnuchin appears to be enjoying the trappings of being a cabinet secretary. He meets weekly with Federal Reserve Board Chair Janet L. Yellen, often for breakfast or lunch, to discuss a variety of financial market issues.

His wife, actress Louise Linton, has accompanied him to at least two congressional hearings, an unusual occurrence.

Whereas Lew seemed to eschew all the security and publicity — he once stood alone at night in Union Station waiting for his wife to get off a train — Mnuchin travels differently. He was recently seen leaving a Washington custom tailor shop in the middle of a workday with a group of Secret Service agents. His wife gave an interview to Town & Country magazine detailing all the types of diamonds and pearls she would wear at their June wedding.

Mnuchin has made clear that a tax overhaul is a focus of the president, tied to a broader goal of growing the economy at a rate of 3 percent a year, compared with 1.6 percent last year.

Economists say that is unlikely in any sustainable way — and roundly agree that, if the debt limit isn’t increased, the economy will begin contracting, not expanding.

But on the signed news clipping in Mnuchin’s office, Trump made clear that even growing the economy at a moderately faster pace would not be sufficient. Right after he wrote how proud he was of Mnuchin, he added, “5% GDP.”