A combination photo shows US Democratic presidential candidate Hillary Clinton (L) and Republican US presidential candidate Donald Trump (R) in Los Angeles, California on May 5, 2016 and in Eugene, Oregon, US on May 6, 2016 respectively.
WASHINGTON – Donald Trump loves to brag about how rich he is. One question swirling through the US presidential campaign now is this: has he managed to avoid paying income taxes?
Trump, a billionaire who steadfastly refuses to release his tax returns – a four-decade tradition among candidates for the White House – raised eyebrows with an odd remark during Monday's debate with Hillary Clinton.
Clinton, the Democrat, offered several hypotheses as to why Trump, the Republican, might be declining to make that information public: maybe he is not as rich or as charitable as he makes himself out to be.
Or, she mused, "maybe he doesn't want the American people, all of you watching tonight, to know that he's paid nothing in federal taxes".
With an estimated 84-million people watching the debate, Trump quickly interjected, "that makes me smart".
A few minutes earlier he had said he filed a financial disclosure statement with the Federal Election Commission in which he reported $694-million (R9,5-billion) in revenue last year.
Filing such forms is mandatory for presidential hopefuls, but they are less detailed than a full-blown tax return.
After the debate Trump seemed to backtrack as he told reporters "of course I pay federal taxes".
"I hate the way our government spends our money," he added. "They throw it out of the window."
For months the Democrats have been demanding that Trump release his tax returns, as Clinton and her husband Bill have done for years.
So the Democrats pounced on Trump's comment about being smart when it comes to paying – or not paying – Uncle Sam.
"He acknowledged that he didn't pay taxes... because he's smart," Vice President Joe Biden said on Tuesday while campaigning for Clinton at Drexel University in Philadelphia.
"Tell that to your mothers and fathers who are breaking their neck to send you here. They're paying taxes," Biden said.
"It angers me. It angers me."
A recent Quinnipiac poll found that 75 percent of Americans want Trump to come clean on his taxes – 60 percent among Republicans, 92 percent among Democrats.
Trump argues that he does not release the data because he is being audited by the Internal Revenue Service. But the IRS says being under audit does not mean someone cannot release their returns.
Trump's son Donald jr says the document is 12,000 pages long.
Some experts say Trump has probably used every possible tax break available to real estate developers like him.
The US tax code provides for generous write-offs at the start of business projects, and these are particularly sweet for real estate developers and investors.
"His tax returns will show us how far he pushed the envelope to turn ordinary income into capital gains, defer the payment of tax through 'like-kind exchanges' and otherwise create an impenetrable maze for IRS auditors," Jack Blum, of the not-for-profit Tax Justice Network USA, told Marketwatch recently.
Such exchanges are transactions that allow for the sale of one asset and acquisition of a similar one without causing immediate tax liability on the disposal of the former.
The only glimpse into how Trump reports his income goes back to the 1970s, when he applied for a casino license in New Jersey.
Trump paid more than $71,000 in federal income taxes on about $218,000 of taxable income earned from 1975 to 1977, according to the New York Times.
But thanks to deductions granted to developers and losses from partnerships, Trump reported "negative income" of $406,379 in 1978 and $3,4m in 1979, the Times added.
So he owed no taxes for those two years. At that time Trump said he was worth hundreds of millions of dollars.
Citizens for Tax Justice says Trump may write off most of his lavish lifestyle as business expenses.
Trump says he is worth around $10bn. Forbes magazine puts the figure at $4,5bn.