Trump: Why doesn't Romney endorse Rubio already?

Republican presidential front-runner Donald TrumpDonald TrumpSenate passes stopgap funding bill, averting shutdown Critics of Jeff Sessions's LGBT case don't know their history (or his) Senate advances funding measure, avoiding shutdown MORE is increasing his attacks on Mitt Romney, questioning why the 2012 GOP nominee has not yet endorsed Marco RubioMarco RubioSenate clears water bill with Flint aid, drought relief What Trump's Cabinet picks reveal House passes water bill with Flint aid, drought relief 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.”