GOP Rep. Darrell Issa took aim at 2012 GOP nominee Mitt Romney Wednesday, saying he "needs to get over" Donald Trump winning this year's nomination and help the party unite before November.

Appearing on CNN, Issa went after Romney hours after the former nominee said that Trump may have a "bombshell of unusual size" sitting in his tax returns, which Trump has refused to release while being audited.

"He needs to get over the fact that, in fact, somebody he didn't pick won," Issa told CNN's Wolf Blitzer. "I was a Marco Rubio person, I was on the campaign trail for four months. You know, Donald Trump's a good man. He wasn't my first choice. Mitt Romney's in the same boat.

"We need to unify, and I think we're coming together. It is important that we all come together," Issa continued. "We've come together over more controversial candidates in the past, including Gerald Ford and, actually, [George] H.W. Bush after the largest tax increase in American history."

Earlier Wednesday, Romney released a statement taking Trump to task for his refusal to release his tax returns, saying it is "disqualifying" that the real estate mogul refuses to do so.

Trump reignited conversation about his tax returns when he told the Associated Press that there is "nothing to learn from them." However, Trump later tweeted out that he would release his taxes when the audit is complete.

Former Secretary of State Hillary Clinton also hit Trump for his comment to the AP while in New Jersey, telling supporters that "we're going to find out" why he's not releasing them.