So much for “so much winning.” Donald Trump’s big, beautiful wall is still a fantasy. Repeal and Replace has failed three times. With G.O.P. control of both houses, nine months into his administration, Trump has suffered a staggering number of losses, and has virtually no legislative accomplishments. So everything is riding on tax reform—it’s Trump’s last, best hope to save his first year in office. “This is really the primary legislative item that the president wants to accomplish,” Jason Miller, a former senior communications adviser for the Trump campaign who is now a managing director at Teneo Holdings, told me. Gary Cohn, the president’s senior economic adviser, called it a “once-in-a-lifetime opportunity.”

It might cause some pause, then, to recognize that other than Donald Trump, the person most responsible for making this grand ambition a reality is Steven Mnuchin. A day after Trump flew to Indianapolis to announce his tax proposal, Mnuchin, the Trump administration’s main spokesman for the plan, took to the stage in Washington to sell it. The proposal, which was hashed out by the so-called Big Six, which includes the leadership of both the House and Senate as well as Mnuchin and Cohn, added few details to the one-page document the administration released over the summer. The plan proposes to cut the corporate tax rate to 20 percent from 35 percent. “People spend a lot of time trying to figure out how to get around taxes when they’re 35 percent. There’s no question that companies spend too much time trying to figure out how to keep money offshore,” Mnuchin said in Washington the day after Trump announced the plan. He added that the plan would “create enormous capital investment and enormous jobs and revenues.” Mnuchin has had to back off his initial assurance during his confirmation hearing that “there would be no absolute tax cut for the upper class.”

Cohn and Mnuchin are both Goldman Sachs alumni of impressive personal wealth. But past that, their resemblance is less close. Cohn climbed aggressively to the top of the firm, with sharp elbows and impressive intelligence. “Gary is much smarter than Steve,” one former Goldman partner told me.

Mnuchin’s father, Robert Mnuchin, was also a partner at Goldman Sachs and a legendary equities trader who established the bank’s block-trading business. (The senior Mnuchin also sported an Amish farmer’s beard, an unlikely look among Goldman’s partners.) After he made his fortune, the older Mnuchin founded the Mnuchin Gallery on the Upper East Side of Manhattan, dealing artists like Jackson Pollock, Frank Stella, and Willem de Kooning, the kind of work that attracted New York’s most sophisticated elites. Mnuchin grew up knowing most of A-list Manhattan. He attended Riverdale Country School and later went to Yale, where he was the publisher of the Yale Daily News and a member of Skull and Bones.

At Goldman, where Mnuchin eventually became chief information officer, he didn’t exactly command attention. Another former Goldman partner recalled Mnuchin as “a bit of a cipher. His father was a towering figure. Poor Steve had to walk in his shadow. He was just not that impressive.” One former Goldman partner summed up his impressions of Cohn and Mnuchin in this way: “Gary’s was one of the most graceful exits given he had no other options. Steve Mnuchin we are struggling to remember.”

After Goldman, Mnuchin moved into the hedge-fund business, including as vice chairman of ESL Investments, owned by his Yale roommate Eddie Lampert. He founded Dune Capital Management with two Goldman colleagues. Partly with his father’s money, he became a significant Hollywood financier, helping to finance Avatar, American Sniper, Magic Mike XXL, and Wonder Woman, among other films. He literally wandered into Trump’s campaign when he stopped by a victory party in Trump Tower celebrating a primary win in New York. Mnuchin has told the story of how all of a sudden he was invited on stage by then-candidate Trump. Soon after, he was the campaign’s finance chair.

Gary Cohn began as the brains of the operation, such as it is, the most important architect of the tax-reform plan and most of the president’s economic vision—but that was before Charlottesville, when Cohn publicly castigated the president for his remarks. Since then, he’s been largely shunned—the president, reportedly, wouldn’t look him in the eye. A top administration official told me that Cohn is still in regular contact with Trump and very much a part of the tax-reform effort.

“Secretary Mnuchin is a key player in the tax talks, but both Washington and Wall Street generally view Cohn as the administration’s point person,” said Isaac Boltansky, director of policy research at Compass Point, a boutique D.C. investment bank focused on the financial services sector.” This is partly due to the fact that Cohn “hit the ground running right after the inauguration. Secretary Mnuchin, on the other hand, wasn’t in his chair until mid-February and there are a considerable number of empty seats at the Treasury Department,” Boltansky added.

Meanwhile, Mnuchin, always one of the most publicly sycophantic of the president’s advisers, has doubled down on his support—he was the West Wing official who defended Trump most vigorously in his feud with the N.F.L., saying the players could exercise their First Amendment rights on their own time. Mnuchin’s defense of the president’s positions has made him the butt of late-night television. His defense of the president’s statements on the N.F.L. was satirized by The Daily Show’s Trevor Noah, who said, “It’s almost like he’s the ventriloquist and the dummy at the same time.”

This seems to have had the effect of deepening his personal relationship with Trump and augmenting his role. As reported in The Wall Street Journal and confirmed by a top White House official, Mnuchin is sitting in on meetings to vet potential future Fed chairs, a job that once seemed all but promised to Cohn. “When Steve talks you can have a very high degree of certainty, you can be nearly sure he has discussed what he is going to say with the president,” Miller said, “or that it is something the president has articulated and he wants to communicate it through Secretary Mnuchin.” Miller told me that while Cohn has been deeply involved in coming up with the outlines of the tax proposal, because of Mnuchin’s friendship with Trump, “[He] has that deeper fundamental understanding of what the president is trying to do.”

In truth, neither Mnuchin nor Cohn could be an effective salesman for tax reform, beginning with their enormous wealth, which has led to the occasionally epic tone deafness on display in the last few weeks. Cohn seems to have forgotten how much anyone in the middle class makes. Mnuchin, worth an estimated $300 million, requested to take a government jet on his honeymoon to Europe this summer, according to ABC. The request was later rescinded after helpful colleagues pointed out to Mnuchin that it might look bad if someone of his net worth was charging taxpayers several hundred thousand dollars to fly him and his new wife to Europe. The wedding of Mnuchin and Louise Linton was officiated by Vice President Mike Pence and attended by various Cabinet members, including Jared Kushner and Ivanka Trump. Linton previewed the occasion by sitting down with Town & Country to discuss all the diamonds she would be wearing for the celebration.

And Mnuchin is as handicapped as any administration official in dealing with Congress. House Republicans were turned off by Mnuchin’s pitch for them to vote for a three-month debt and spending extension, exacerbating divisions between Capitol Hill and the White House. “His performance was incredibly poor, and his last words, and I quote, were ‘vote for the debt ceiling for me,’” Rep. Mark Walker (R-N.C.), chairman of the conservative Republican Study Committee (R.S.C.), a group that opposed the bill, told The Hill earlier this month. “He doesn’t get politics and doesn’t have any relationships,” Stan Collender, a former staffer on the House and Senate Budget Committees and a current executive vice president of MSLGROUP in Washington, told me. “He doesn’t know how to operate in a world where members of Congress are an equal branch of government to the White House. He’s adopted some of Trump’s attitude that Congress is a wholly owned subsidiary of the White House.”

Collender said that on the Hill, Mnuchin is seen as “a political novice who doesn’t have the substantive background to do the job he’s being asked to do,” like predecessors Robert Rubin or Don Regan. A senior administration official told me that the White House is confident that building out dedicated tax-reform teams covering communications, strategy, coalitions, and public liaison functions will help guarantee that tax reform will be more successful than health-care reform efforts. But given Mnuchin’s rolling deadlines, it’s hard to see him as having a handle on the process. “This won’t happen fast if it happens at all,” Collender told me. “By the end of the year he could be very close to being in the same position that [recently resigned H.H.S. Secretary] Tom Price found himself in.” Price resigned after undergoing scrutiny of his use of private flights, and after suffering Trump’s disappointment over the repeated failures of Repeal and Replace.

Mnuchin also took a private government plane to Kentucky, where he viewed the total eclipse with his wife. (When questioned recently about whether he regretted the flight, Mnuchin said no.) Mnuchin and Linton were spotted over the weekend at table one at Cafe Milano in D.C. But tax reform’s largest obstacle is almost certainly not Steve Mnuchin. “A lot of how this is likely to happen is completely based on Trump,” Terry Sullivan, Marco Rubio’s former campaign manager and a current partner at Firehouse Strategies in Washington, D.C., told me. “He undercuts the people he sends to negotiate on a daily basis. Why do you take anything that Steve Mnuchin or Gary Cohn says to the bank?” When Trump made a deal with Democratic leaders Nancy Pelosi and Chuck Schumer to extend the debt limit, “His senior policy people and his chief of staff didn’t know he was going to do that,” Sullivan added. “To that extent, [Trump] hobbles anyone and everyone’s ability to work for him.”

“The second public sentiment seems to be turning against any deal his team put together,” warns Republican political strategist Sullivan. “[Trump]’s going to pull the rug out from underneath them.”