The California billionaire Don Hankey made his fortune by offering high-interest auto loans to buyers with bad credit. This week, he signed up a new client in need of urgent help: Donald J. Trump.

A company operated by Mr. Hankey came to the former president’s aid on Monday and provided a $175 million bond in Mr. Trump’s New York civil fraud case, helping him avoid financial disaster. It was the first time one of his companies had done such a deal.

Mr. Hankey earned a reputation as a provider of risky and lucrative loans, specifically collateral-based debt that can be arranged more quickly than conventional loans and requires borrowers to pledge valuable assets. His companies are known for relentlessly calling people who miss payments by a day, and repossessing vehicles from delinquent borrowers, according to news accounts and regulators.

Now, Mr. Hankey’s highest-profile client is running again for the White House and, if elected, could exert influence over federal regulation of his high-interest loan industry.

Mr. Hankey, 80, is in the public eye thanks to a New York judge’s decision in February that Mr. Trump was liable for conspiring to manipulate his net worth. The judge imposed a $454 million judgment, a monumental penalty that jeopardized his business empire and raised the possibility that the state could seize some of Mr. Trump’s buildings.

The former president appealed the judgment, and in the meantime a panel of five appellate court judges decided last week that Mr. Trump could post a far lower bond, $175 million.

Mr. Hankey said it had been an easy decision to provide the bond, a legal promise to cover a portion of the penalty if Mr. Trump loses his appeal and fails to pay. People seeking such bonds generally pay a fee to a bonding company and pledge collateral including as much cash as possible for the full amount of the judgment. Mr. Trump’s $175 million in cash collateral is being handled on his behalf by a brokerage firm, which could invest it and gain interest for him, Mr. Hankey said.

Mr. Hankey said he did not mind how it was invested as long as it could be liquidated into cash on short notice.

“I looked at it as business, No. 1,” Mr. Hankey said in an interview on Wednesday, “and No. 2, I was happy to help someone.”

While Mr. Hankey’s companies had not previously provided an appellate bond, Mr. Hankey said the deal was not dissimilar from others underwritten by Knight Insurance Group, one of eight subsidiaries of the Hankey Group. Mr. Hankey said he did not know the origins of Mr. Trump’s cash but believed it was all his money.

He insisted that the arrangement was not motivated by politics, but that he had supported Mr. Trump before and would do so again in November. Mr. Hankey said he believed the judgment in the civil fraud case was “a bit unfair.”

“We could care less if someone is a Republican or a Democrat,” he said.

Like Mr. Trump, Mr. Hankey owns companies that have run afoul of regulators and have been accused of deceptive business practices.

In 2015, the Consumer Financial Protection Bureau ordered two of his companies, Westlake Services L.L.C. and Wilshire Consumer Credit, to refund customers $44 million and pay a $4.2 million fine for deceiving customers. In 2022, another firm, Westlake Financial, agreed to pay $225,000 to settle allegations that it had, in violation of federal law, repossessed at least 70 vehicles from members of the United States military who had been called up for active duty.

Mr. Hankey said his companies had corrected past practices. “We have 1.5 million customers, so it’s hard not to make mistakes, but obviously we try to minimize the mistakes we make,” he said.

Richard Painter, who served as the chief ethics lawyer to President George W. Bush and ran as a Democrat for Congress, said he was “very, very concerned” about a major player in the loan industry helping Mr. Trump and the lack of details about the bond, including whether other people were involved in providing the cash collateral.

Mr. Painter said he was worried that Mr. Hankey could ask for favors from Mr. Trump if he were re-elected. “He has got a lot of leverage,” Mr. Painter said, “and if Donald Trump is back in the White House, we know exactly what he wants, and that is for Trump to just shut down C.F.P.B.”

Mr. Hankey said that he would not lobby Mr. Trump if he were president. “Regulation is fine if it’s not overdone,” he said.

Like Mr. Trump, Mr. Hankey is a billionaire, according to Forbes, with wealth estimated at $7.4 billion. His companies are based in Southern California and include Westlake Financial, one of the country’s largest providers of subprime and near-subprime loans; North Hollywood Toyota, a dealership in Los Angeles County; and Hankey Investment Company, a real estate firm.

Forbes has called Mr. Hankey the “little-known king of subprime car loans.”

His offer to help Mr. Trump came together over several weeks after the New York judge issued his final ruling in February ordering the former president to pay the $454 million judgment. Mr. Hankey said he had been talking to his wife, Debbi, about the ruling when the conversation shifted to how one of his companies could help.

Ms. Hankey knew someone close to Mr. Trump and made contact with that person, whom Mr. Hankey declined to identify. That led to negotiations between executives at Knight and representatives for Mr. Trump, including Alan Garten, executive vice president and general counsel for the Trump Organization.

While Mr. Trump’s lawyers said in a court filing last month that he faced “insurmountable difficulties” in raising a bond for the full penalty, Mr. Hankey said on Wednesday that his company had been discussing possible terms to secure the full $454 million.

He said that would have carried far more risk, requiring Mr. Trump to provide not only cash as collateral but also some of his real estate.

Kitty Bennett contributed research.