That year, he would later learn, was the beginning of Mr. Trump’s reckoning with a decade of rapid, debt-fueled expansion. The eclectic empire Mr. Trump had built with leverage from his father’s brick-and-mortar fortune began to fail, generating enormous losses and bringing him to the brink of personal bankruptcy.

Image The Castle casino in Atlantic City recorded total losses of $93.2 million in 1990 and 1991. Credit... Michael Ein/The Press of Atlantic City, via Associated Press

The full magnitude of the financial hemorrhaging was a closely held secret until this weekend, when The New York Times published portions of Mr. Trump’s 1995 tax records that showed business losses of $916 million, creating a tax deduction that could have allowed him to legally avoid paying any federal income taxes for up to 18 years.

For a single businessman to declare losses approaching $1 billion is so extraordinary that it caused several accountants and lawyers consulted by The Times to blanch. The precise breakdown of that figure — specifically which Trump enterprises were responsible for how much — remains murky, hidden in a schedule attached to Mr. Trump’s returns that has not become public. But a review of public records and interviews with those who were present makes clear that it was decisions Mr. Trump made at the helm of his business empire during the 1980s that led to its nearly imploding.

Mr. Trump, the Republican nominee for president, portrays himself now as a self-made man who began life with what he has characterized as a meager $1 million advance from his father. That figure itself represents a significant understatement about the support his father provided him over the years. But in his darkest moment, Mr. Trump again leaned on his family’s wealth, this time to ride out a financial tsunami.

At one point, the balance of his personal bank accounts was expected to drop below $1 million.

By 1990, Mr. Trump had amassed $3.4 billion in debt, much of it in the form of high-interest junk bonds. He was personally liable for $832.5 million of that. He had bought a yacht for $29 million, the Plaza Hotel in Manhattan for $407 million and a failing airline for $365 million. All were losing money.