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Donald Trump continues to double down on controversial remarks he made during the first presidential debate Monday night, giving his rival, Hillary Clinton, and her supporters more ammunition to attack.

Much of the attention has gone to his disparaging comments about former Miss Universe Alicia Machado, but Trump’s contention that he has simply been "smart" to pay little to no federal income tax has quickly become a fixture in Democrats’ anti-Trump playbook -- one aimed directly at middle-class voters.

When Clinton accused Trump of paying no income taxes at the debate, Trump said, "that would make me smart."

Trump has yet to deny that he does not pay taxes - or affirm that he does.

Instead he told Fox News Host Bill O’Reilly Wednesday night that not paying taxes “would make me smart because tax is a big payment.”

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Trump has refused to release his most recent years’ tax returns, blaming an IRS audit. But he hasn’t released earlier years that are no longer under audit, either. Reporters have found five years’ worth of Trump’s tax returns and he paid no federal income taxes two of those years.

As Trump continues to defend his comments, the Clinton campaign has more incentive to use it against him. And they are.

Related: Donald Trump's Campaign is Quietly Shouting About Clinton Sex Scandals

In four out of five campaign events since the debate, Clinton and her surrogates have woven Trump’s tax comment into their stump speeches, mocking the major party presidential candidate who is worth at least hundreds of millions of dollars and perhaps billions and advocated not paying taxes.

"I think there's a strong possibility he hasn't paid federal taxes in a lot of years," Clinton said in Raleigh, N.C. the day after the debate. "If not paying taxes makes him smart, what does that make all the rest of us?"

First Lady Michelle Obama used the attack twice in Pennsylvania Wednesday.

“So if a candidate is erratic and threatening, if a candidate traffics in prejudice, fears, and lies on the campaign trail, if a candidate thinks that not paying taxes makes you smart,” don’t trust the candidate, Obama said in Philadelphia and in Pittsburgh.

And Vice President Joe Biden knocked Trump as well – also in Philadelphia.

“He acknowledged that he didn't pay taxes because he said ... because he's smart; makes him smart," Biden said Tuesday.

The Clinton campaign released a digital video from the debate using that clip with that caption that Trump is someone “who will fight for himself.”

And the super PAC backing Clinton, Priorities USA, released a digital ad on social media platforms in four states Clinton needs to win - Ohio, North Carolina, Florida and Pennsylvania, playing the same clip from the debate. The six-figure digital ad is part of the super PAC's $35 million digital strategy.

“We plan to continue holding Donald Trump accountable for the fact that he stiffed our troops, veterans, schools and more by paying zero dollars in in federal income taxes and then having the audacity to brag about it,” Justin Barasky, spokesman for Priorities USA, said.