Hillary Clinton has torn into her opponent Donald Trump over his possible use of deductions from business losses in the 1990s to avoid paying income taxes for several years.
"Trump represents the same rigged system that he claims he's going to change," Clinton said at a campaign event in Ohio.
It came two days after the New York Times broke the story about a 1995 income tax document it obtained and the tax benefits the Republican presidential nominee possibly took advantage of.
Tax experts hired by the Times and who analysed the three-page document said that tax rules advantageous to wealthy people would have allowed Trump legally to use a $US916 million loss to cancel out an equivalent amount of taxable income over an 18-year period.
"What kind of genius loses a billion dollars in a single year?" Clinton said.
"He's taken corporate excess and made a business model out of it. He abuses his power, games the system, puts his own interests ahead of the country's," she said. "It's Trump first and everyone else last."
The story said the tax documents showed the loss came from the financial mismanagement of casinos in Atlantic City, New Jersey, his attempt to start an airline and his purchase of a large hotel in Manhattan.
Trump, speaking at a campaign rally in Colorado, said he had a responsibility to pay as little tax as legally possible.
"I have legally used the tax laws to my benefit and to the benefit of my company and my investors and my employees," he said.
On Sunday his campaign issued a statement defending Trump for paying hundreds of millions of dollars in other taxes, including property taxes and sales and excise taxes.
Trump is also facing more controversy over his charitable foundation, which the attorney general of New York said violated New York laws by not properly registering as a charity with the state.
The state has ordered the foundation to stop soliciting contributions because it was doing so without having properly registered with the state's Charities Bureau to accept contributions or providing annual financial statements.
The latest clash comes as Clinton gained a five-point lead over Trump after last week's first presidential debate, according to an opinion poll published on Monday.
Clinton led Trump by 47 per cent to 42 per cent among likely voters, the CNN/ORC poll said, with most of her gains apparently coming from men and independents.
