But the market for life settlements largely collapsed during the financial crisis.
Eventually, Dune, like many hedge funds during the worst of the crisis, faced investor withdrawals. Mr. Mnuchin and one of his co-founding partners, Chip Seelig, decided to wind down the operation. The real estate arm of Dune was spun off into a firm led by Dune's third co-founder, Daniel M. Neidich.
After the split, Mr. Mnuchin and Mr. Seelig set up shop in Southern California. Both became big players in movie financing, lending money to hits like "Avatar" and "Gravity" along with flops like "In the Heart of the Sea."
But it was the deal for a failed California bank, IndyMac, that contributed mightily to Mr. Mnuchin's personal fortune and the one deal that has created the most political heat. On Friday, Senate Democrats, in a sign that they intend to go hard after Mr. Mnuchin, set up a website that calls him the "foreclosure king," and asked customers of OneWest to submit complaints in advance of the confirmation hearing.
In late 2008, Mr. Mnuchin joined with Mr. Paulson to put together the OneWest deal. They were part of the initial round of bidding for the assets of IndyMac, which the Federal Deposit Insurance Corporation had seized that summer. The group, which also included Mr. Soros and other investors, won the deal with a $1.55 billion bid.
One of the competing bids was put together by J. Tomilson Hill, head of the Blackstone Group's hedge fund and asset management group, and Goldman.
"As a deal person, I had nothing but admiration for how entrepreneurial he was and how quickly he adapted to the circumstances," Mr. Hill said in a recent interview. He was one of the first to call Mr. Mnuchin to congratulate him on his new job as campaign finance chairman.
The remnants of IndyMac reopened under the name OneWest with Mr. Mnuchin as chairman and chief executive. But controversy followed, including complaints over foreclosures on soured mortgages that IndyMac had written before the crisis. In particular, it drew scrutiny for its business in reverse mortgages — products pitched to older people who had paid off their initial mortgages but needed cash.
The Department of Housing and Urban Development is investigating complaints about the foreclosure practices of OneWest's reverse mortgage business, which resulted in 16,000 foreclosures alone.
CIT Group, which merged with OneWest last year in a $3.4 billion deal that provided a hefty payout for Mr. Mnuchin and his co-investors, has said it is trying to resolve the HUD inquiry. It is also dealing with a $230 million charge the company said in July it had to take in connection with accounting issues from the reverse mortgage business.
An analysis in 2015 presented by the California Reinvestment Coalition, which lobbied against the merger, found that 68 percent of the 36,000 foreclosures in California by OneWest, including those on reverse mortgages, occurred in communities that were primarily nonwhite. A government filing shows OneWest offered to modify about 101,000 mortgages for its customers.
Mr. Mnuchin stepped down from the helm of OneWest in March, not long before he became among the first in finance to come out for Mr. Trump and against Hillary Clinton. Mr. Keller said Mr. Mnuchin was "proud of his record at OneWest."
Still, the worlds of finance and politics can be small ones, and Mr. Mnuchin even has a connection to Mrs. Clinton. The in-house accountant at Goldman who prepared a tax filing for Steven T. Mnuchin Inc. many years ago has among her clients, at her current accounting firm, the former Democratic presidential candidate.