The party's former presidential nominee said he is suspicious because every time Trump is asked about his taxes, he "dodges" and "delays".
"Frankly, I think we have a good reason to believe that there's a bombshell in Donald Trump's taxes," Romney was quoted as saying by Fox news.
"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 to the disabled like he's been telling us he's doing," Romney said.
"We're talking about taxes already filed, back taxes. And my back taxes, when I ran in 2012, my back taxes I put out in January of 2012," Romney said.
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"You know, an awful lot of people speculate that he hasn't made nearly as much money as he said, who knows, because he doesn't release his tax returns," he said.
Trump, who has won three of the four GOP presidential primaries so far, hasn't released his tax returns yet. Cruz has already released his tax returns.
"The Democrats and are an awful lot of the mainstream media is using kid gloves on Donald right now because they want Donald to be the nominee. And the instant he is the nominee, they will unleash every cannon they can to elect Hillary Clinton," he said.
"We can't risk that because if we do, we are going to lose the country, we are going to be buried in debt, we'll lose the Supreme Court. We can't let that happen," Cruz said.
Trump, however, in his characteristic way slammed Romney and described him as a fool.
"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.