TRUMP: Mitt Romney was 'one of the dumbest' candidates in GOP history

Republican presidential candidate Donald Trump continued to lash out at Mitt Romney on Thursday morning.

"Mitt Romney, who was one of the dumbest and worst candidates in the history of Republican politics, is now pushing me on tax returns," Trump wrote on Twitter.

"Dope!" he added.

Look back on Romney's campaigns for president:

The day before, Romney said in a Fox News interview that Trump and other GOP candidates should release their tax returns.

Romney, the GOP nominee in 2012, suggested that there was a "bombshell" hidden somewhere in Trump's tax documents:

Well, I think there's something there. Either he's not anywhere near as wealthy as he says he is, or he hasn't been paying the kind of taxes we would expect him to pay. Or perhaps he hasn't been giving money to the vets or the disabled, like he's been telling us he's been doing.

Romney was famously dogged by questions over his own tax returns in the 2012 race. Trump, a billionaire real-estate developer, has insisted that his tax filing is simply too complicated to release quickly.

Trump responded to Romney's interview by calling him a "fool" and "goofy" on Twitter. On Wednesday night, he also retweeted a small flurry of supporters using insults like "pathetic" and "jealous" to describe Romney:

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