REUTERS/Jim Young
U.S. Republican presidential candidate Donald Trump speaks at a campaign rally in Las Vegas, Nevada February 22, 2016.
Just over an hour after Romney questioned Trump's personal finances in a Fox News interview Wednesday, Trump took to Twitter to criticize the 2012 Republican nominee.
"Mitt Romney, who totally blew an election that should have been won and whose tax returns made him look like a fool, is now playing tough guy," Trump said in a tweet.
Trump also recalled his endorsement of Romney in 2012, saying he "should have known" Romney would lose to President Barack Obama.
"When Mitt Romney asked me for my endorsement last time around, he was so awkward and goofy that we all should have known he could not win!" Trump tweeted.
During his interview on Fox, Romney questioned the details of Trump's tax returns. And he added his voice to those speculating that Trump's worth may not be as high as he claims it is.
"We have good reason to believe that there's a bombshell in Donald Trump's taxes," Romney told Fox News host Neil Cavuto on Wednesday.
"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 continued.
Romney called on several major Republican presidential candidates to release their tax returns. But he took particular aim at Trump.
"Donald Trump and Marco Rubio and Ted Cruz have not shown us their back taxes," Romney said. "This was an issue on my campaign."