Donald Trump avoided paying potentially hundreds of millions of dollars in taxes in a way even his own lawyers considered questionable, according to a report.
The New York Times said the manoeuvre may also explain how the Republican presidential candidate posted a one-year loss of more than $900m a few years later, enabling him to avoid paying federal income taxes for perhaps 18 years.
At issue is how billionaire property tycoon Mr Trump was able to cancel hundreds of millions of debt as his casino empire in Atlantic City went broke in the early 1990s.
Cancelled debt is generally treated as taxable income, meaning Mr Trump would have owed the Internal Revenue Service significant money on debt that his creditors forgave.
The newspaper said it obtained documents from 25 years ago showing Mr Trump essentially traded the debt relief for nearly worthless "partnership equity" to avoid any tax liability.
In practice, the strategy was almost identical to a tax move that was already outlawed, but differed in minor details. Mr Trump's own lawyers advised him the IRS would be likely to find it improper if he were audited, the paper said, and Congress explicitly outlawed the manoeuvre in 2004.
Mr Trump's Democratic White House rival Hillary Clinton, then a New York senator, was among those who voted to close the loophole.
Hope Hicks, Mr Trump's spokeswoman, told the Times that its reporting "suggests either a fundamental misunderstanding or an intentional misreading of the law".
She said Mr Trump did not think taxpayers "should file returns that resolve all doubt in favour of the IRS".
Meanwhile Mr Trump plunged into his final-week sprint to election day by unleashing a harsh new attack against Mrs Clinton in Michigan, a state that has not favoured a Republican for president in nearly 30 years.
His message was welcomed by supporters, but his location frustrated anxious Republicans who fear their nominee is riding his unorthodox political playbook too long - even as Mrs Clinton's developing email problems offer new political opportunity.
"Her election would mire our government and our country in a constitutional crisis that we cannot afford," Mr Trump declared in Grand Rapids, pointing to the FBI's renewed examination of Mrs Clinton's email practices as evidence the former secretary of state might face a criminal trial as president.
National polls show a tightening race but with more than 23 million ballots already cast through early voting, it is unclear whether Mr Trump has the time or capacity to dramatically improve his standing over the next week in states like Michigan, where few political professionals in either party expect a Republican victory on November 8.
Mrs Clinton, defending herself from the new FBI examination, focused on battleground Ohio, a state Mr Trump's team concedes he must win.
"There is no case here," she insisted. "Most people have decided a long time ago what they think about all this."
Later Clinton campaign manager Robby Mook decried what he called a "blatant double standard" following a CNBC report that FBI director James Comey opposed releasing details about possible Russian interference in the US election because it was too close to election day.
Mr Comey issued a letter to congressional leaders on Friday about the FBI's renewed interest in Mrs Clinton's email.
The FBI declined to comment on Monday.
Amid the attacks and counter-attacks, the race for the White House remains at its core a test of a simple question: Will the conventional rules of modern-day campaigns apply to a 2016 election that has been anything but conventional?
For much of the year, Mrs Clinton has pounded the airwaves with advertising, assembled an expansive voter data file and constructed a nationwide political organisation that dwarfs her opponent's.
She and her allies in a dozen battleground states have more than 4,800 people knocking on doors, making phone calls and otherwise working to support her candidacy.
Mrs Clinton's numbers, as reported in recent campaign filings, tripled those of Mr Trump and the national and state Republican parties.
The New York businessman over the past year has largely ignored the key components of recent winning campaigns, depending instead on massive rallies and free media coverage to drive his outsider candidacy.
This week he is devoting his most valuable resource - his time - to states where polls suggest he is trailing Mrs Clinton by significant margins.
Mr Trump had two rallies on Monday in Michigan, a state that last went for a Republican presidential nominee in 1988. The day before, he appeared in New Mexico, which has supported the party just once over the last three decades.
And today, he is due to appear with running mate Mike Pence in Wisconsin, which has not backed a Republican for president since Ronald Reagan's re-election in 1984.
"It makes no sense to me," Republican pollster Frank Luntz said of Mr Trump's strategy. But Michigan-based Republican operative Saul Anuzis described the state as "a creative opportunity" for Mr Trump.
"The demographics in Michigan are perfect for Trump," Mr Anuzis said of the state's large white working-class population. "That doesn't mean he'll necessarily win here."
Overall, more than 23 million votes have been cast, far higher than the rate in 2012, according to Associated Press data. That represents nearly 20% of the total votes expected nationwide, if turnout is similar to 2012.
In all, more than 46 million people - up to 40% of the electorate - are expected to vote before election day.
In Colorado, Democrats lead Republicans by three percentage points in early voting, reversing a trend in the past two elections in which Republicans led in early voting and large numbers of Democrats voted on election day.
In swing state Iowa, Republicans trail Democrats in early voting as well, though by a smaller margin than four years ago. Both parties are well behind where they were four years ago.
Meanwhile, some Republicans are sceptical that the FBI's renewed interest in Mrs Clinton's emails will erase the Democrat's advantage.
"It would take something like an indictment to turn it into a dead heat," Republican pollster Whit Ayres said.
As for Trump's charge that a Clinton election might prompt "a constitutional crisis," the Justice Department's office of legal counsel said in 1973 that criminally prosecuting a president would unconstitutionally undermine the executive branch.
A 2000 memo reached a similar conclusion, but presidents can, however, face civil lawsuits.