Donald Trump boasted during Monday night's presidential debate that some of his previously released federal tax returns showed that he paid zero percent in federal taxes.

'That makes me smart,' the businessman shot back when rival, Democrat Hillary Clinton, brought up the point.

After Trump initially said he'd release tax returns against his attorneys' wishes if Clinton released some of her deleted State Department emails, the former secretary of state went in on him for pulling a 'bait and switch'.

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Republican Donald Trump replied that it makes 'him smart' that he paid no federal income tax

Former Secretary of State Hillary Clinton pressured Donald Trump to release his tax returns during tonight's presidential debate

Clinton named the reasons why Trump may be reluctant to let the American public see his returns, which he's repeatedly said he's not releasing because he's under a 'routine audit.'

'First, maybe he's not as rich as he says he is,' Clinton said.

'Second, maybe he's not as charitable as he claims to be,' she continued.

'Third, we don't know all of his business dealings, but we have been told through investigative reporting that he owes about $650 million to Wall Street and foreign banks,' she added.

Her next point got a response out of The Donald.

'Or maybe he doesn't want to the American people, all of you watching tonight, to know that he's paid nothing in federal taxes,' she said.

The Republican businessman then labeled himself 'smart.'

Clinton continued.

'That means zero for troops, zero for vets, zero for schools or health,' she said.

'And I think probably he's not all that enthusiastic about having the rest of our country seeing what his reasons are because it must be something really important, even terrible, that he's trying to hide,' Clinton added.

The issue came up again when Trump was calling out politicians like Clinton for ballooning the national debt.

'Our country has tremendous problems,' he said.

'We're a debtor nation, we're a serious debtor nation and we need new bridges, airports, schools, new hospitals and we don't have the money because it's been squandered on so many of your ideas,' Trump said.

'And maybe it's because you haven't paid any federal income tax for a lot of years. And the other thing I think is important,' Clinton said.