But newly obtained documents show that in the early 1990s, as he scrambled to stave off financial ruin, Mr. Trump avoided reporting hundreds of millions of dollars in taxable income by using a tax avoidance maneuver so legally dubious his own lawyers advised him that the Internal Revenue Service would likely declare it improper if he were audited.

Thanks to this one maneuver — which was later outlawed by Congress — Mr. Trump potentially escaped paying tens of millions of dollars in federal personal income taxes. It is impossible to know for sure because Mr. Trump has declined to release his tax returns, or even a summary of his returns, breaking a practice followed by every Republican and Democratic presidential candidate for more than four decades.

Tax experts who reviewed the newly obtained documents for The New York Times said Mr. Trump’s tax avoidance maneuver, conjured from ambiguous provisions of highly technical tax court rulings, clearly pushed the edge of the envelope of what tax laws permitted at the time. “Whatever loophole existed was not ‘exploited’ here, but stretched beyond any recognition,” said Steven M. Rosenthal, a senior fellow at the nonpartisan Tax Policy Center who helped draft tax legislation in the early 1990s.

Moreover, the tax experts said the maneuver trampled a core tenet of American tax policy by conferring enormous tax benefits to Mr. Trump for losing vast amounts of other people’s money — in this case, money investors and banks had entrusted to him to build a casino empire in Atlantic City.