Trump: Why doesn't Romney endorse Rubio already?

Republican presidential front-runner Donald TrumpDonald TrumpMcMaster to South Korea: US will pay for missile defense system Comedian Hasan Minhaj blasts Trump, media at correspondents' dinner White House correspondents' chief: 'We are not fake news' MORE is increasing his attacks on Mitt Romney, questioning why the 2012 GOP nominee has not yet endorsed Marco RubioMarco RubioOvernight Defense: Commander calls North Korea crisis 'worst' he's seen | Trump signs VA order | Dems push Trump to fill national security posts What’s with Trump’s spelling mistakes? Boeing must be stopped from doing business with Iran MORE.

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Trump has focused his attention on Romney since the former Massachusetts governor 2012 Republican presidential nominee raised questions Wednesday about Trump's tax returns.
 
"Frankly, I think we have a good reason to believe that there's a bombshell in Donald Trump's taxes," Romney said in an appearance on 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."

Trump responded Wednesday by calling Romney a "fool" and mocked him for "playing tough guy."
 
The billionaire business mogul continued Thursday morning with more name-calling, accusing Romney of being "one of the dumbest and worst candidates in the history of Republican politics."
 
 
Trump said in an interview Wednesday night that he will make a decision in the coming months on whether to release the tax returns.

“My returns are extremely complex, and I’ll make a determination at the right time,” he told CNN. “I’m in no rush to do it.”