“This is a tipping point now,” he said of the national debt. “If we don’t address it in this window, in this presidential debate, we’re not going to deal with it,” he said, urging the Republican Party to embrace fiscal conservatism.

“I think we’re walking our way into one heck of a financial storm,” he said. “There is no discussion of debt, deficit and government spending in Washington these days.”

He also called Mr. Trump’s recent attack on four members of Congress, all progressive women of color, “noxious and weird.”

Mr. Sanford, who supported Mr. Trump in 2016, had been one of his most vocal Republican critics in Congress before losing in a primary last summer to the Trump-endorsed Katie Arrington, who lost in November to her Democratic opponent, Joe Cunningham.

In two stints in the House of Representatives, where he served a total of six terms, Mr. Sanford was regarded as one of the body’s most fiscally conservative members. As governor he went so far as to try to reject $700 million in federal funds sent to his state following the recession.