Washington: Amidst reports that Donald Trump probably pays no tax, Hillary Clinton piled pressure on the beleaguered Republican presidential nominee on Friday – releasing her 2015 tax return, which reveals a total tax payout in excess of 35 per cent.

Over the years Trump has noisily demanded that President Barack Obama release practically everything – save for the size of his underwear. In segueing from New York property mogul to national political figure, Trump campaigned at different times for Obama to release his birth certificate; college applications, transcripts and other documents; and passport records.

But through the 2016 presidential campaign, Trump has stubbornly refused to release his own tax returns, fueling speculation that his affairs are structured to take advantage of generous real estate deductions by which he pays little or no tax; that his claimed donations to charity are bogus; that perhaps he's worth only a fraction of what he claims; that some of his business deals, associates or practices are too dodgy to share with voters; and/or that he has funds stashed in foreign bank accounts.

Or as Mitt Romney, the GOP's 2012 presidential candidate and a searing Trump critic, charges, Trump is bent on hiding a "bombshell of unusual size."