Donald Trump told tonight's Republican debate audience that his company is 'under a routine audit,' so that's why he hasn't yet released his tax returns.

He then immediately went on CNN to air a theory about why he's been targeted year after year by the Internal Revenue Service.

'I'm always audited by the IRS, which I think is very unfair,' Trump told CNN's Chris Cuomo, then suggesting it was 'maybe because of religion.'

'Maybe because of the fact that I'm a strong Christian,' Trump asked when Cuomo asked the GOP frontrunner to elaborate. 'Maybe there's a bias.'

As soon as the theory was floated – which lines up nicely next to allegations that the IRS was targeting Tea Party groups – Trump immediately let his foot off the gas.

'I don't know, you'll have to ask them,' Trump shrugged as he walked through the spin room after the Houston debate. 'But I have friends that are also very rich and are never audited.'

Trump also suggested the audit was standard for a company of his business' size.

'It's a very, very big company,' he boasted to reporters.

Scroll down for video

Donald Trump said he wished to release his tax returns, but was under a 'routine' audit and would release them once the feds were done

Donald Trump used the conversation on tax returns to his 2012 GOP nominee Mitt Romney. Trump said he 'looked like a fool' for delaying the release of his own returns

After Trump's initial claim, he then pivoted on the issue to skewer 2012 Republican nominee Mitt Romney, who suggested this week that Trump could have a 'bombshell' in those returns that could effect the presidential race if the billionaire were to become the GOP's nominee.

'I will say this,' Trump said. 'Mitt Romney looked like a fool when he delayed and delayed and delayed and Harry Reid baited him so beautifully and Mitt Romney didn't file his return until September 21 of 2012 about a month and a half before the election.'

'And it cost him big league,' Trump added.

Reid, like Romney to Trump, said on the Senate floor that Romney hadn't paid taxes in years.

After the debate, Romney took to Twitter to respond writing: 'No legit reason [Donald Trump] can't release returns while being audited, but if scared, release earlier returns no longer under audit,' the former Massachusetts governor wrote.

He also reiterated his first points – that more 'bombshells' could exist.

Trump, on the debate stage, also claimed to be a step ahead of his rivals because he released extensive financial disclosure documents in the run-up to his presidential run.

'It shows that I'm worth over 10 billion dollars,' Trump claimed. 'I've built a great company with very little debt. People were shocked. People in the back, the reporters, were shocked.'

Trump also suggested that these types of filings were more informative than a candidate giving over his or her tax returns.

'You don't learn anything about somebody's wealth with a tax return, you learn it from statements,' the businessman explained.

As for the audit, Trump said he's been audited for approximately a dozen years.

'Ever year they audit me, audit me, audit me,' he said, aghast. 'Nobody gets audited. I have friends who are very wealthy people and they never get audited. I get audited every year.'

'So I can't do it until the audit is finished, obviously, and I think people will understand.'

When conservative radio host Hugh Hewitt asked a follow-up question on Trump's finances, the candidate wasn't having it.

Mitt Romney challenged Donald Trump right back suggesting that he could have released at last some years of his tax returns

Mitt Romney again hinted that something was amiss with Donald Trump's financials because the billionaire had yet to release his tax returns

Donald Trump's main rivals - Ted Cruz and Marco Rubio - both used Trump's audit as a talking point when bring up their own finances

'Are you going to ask anybody else that question?' Trump began. 'Every single question to me? I know I'm here for the ratings, but this is ridiculous.'

He also made it personal.

'First of all, very few people listen to your radio [show], so that's the good news,' Trump aimed at Hewitt. 'Which happens to be true, check out the ratings.'

Trump again articulated that he wanted to release his tax returns.

'We're under a routine audit,' Trump said again.

When rivals Marco Rubio and Ted Cruz – who spent tonight's debate playing whack-a-mole with Trump – got their turn to talk taxes, they turned it back on the billionaire.

'There is nothing really that interesting in them so I have no problem releasing them,' Rubio said, telling the moderators that he would be releasing his returns tomorrow or Saturday.

'And luckily, I'm not being audited this year, or last year for that matter,' Rubio uttered.

Cruz pointed a finger right at Trump's audit.

'I think that would underscore the need to release those returns,' Cruz said.