It's presumptive nominee versus former nominee.
Donald Trump called Mitt Romney "ungrateful" for his support during the 2012 Republican primary. Trump said, "Every single robocall I made, he won that state. Every single speech I made, he won that state, in terms of the primaries."
"I believe I won him, or helped him win five states that he was going to lose in the primaries," Trump continued.
And that support came with personal cost to Trump.
"I raised a lot of money for him. I ruined the carpet in my apartments — I had so many people come. We actually had to have two fundraisers because there were so many people."
The former governor of Massachusetts has been outspoken in his criticism of Trump — warning of "demagogues" in both parties in a commencement address over the weekend. His name is also being floated as a possible third-party candidate.
Trump claimed Romney's campaign manager believed he was "too tough and too controversial" during the 2012 campaign, which was why they didn't use him in the general election.
And that's something the businessman hasn't soon forgotten. "I was rough on Mitt because I didn't think they treated me properly. I helped him, really helped him."
Pressed on if he believed Romney was "ungrateful" for his support, Trump replied, "He was. He was ungrateful. Which is okay. A lot of people are ungrateful. But he was ungrateful. They did not respond accordingly."
In a separate interview, Sen. Jeff Flake, R-Ariz., expressed his own reservations about Trump's candidacy: "Donald Trump was not my first choice or my 17th choice to put it mildly."
But Flake rejected the notion that a lack of support from Republican leaders would be the death knell to Trump's campaign. Instead, he said, it will be Trump's positions on issues like a temporary ban on Muslims entering the country that will "impede" him, "not the fact that some Republican leaders disagree with you."
"I'm not under the illusion"
Addressing concerns that he is not committed to his tax plan that would benefit the upper rungs of the income ladder, Trump admitted that what he has released is "really a tax proposal" and that he's not "under the illusion that that's going to pass."
While Trump said he would "make sure the middle class gets good tax breaks" he added that taxes for the wealthy are "going to go up. And you know what, it really should go up."
Trump tried to clarify his tax plan comments, explaining that when it comes time for him to negotiate with Congress, his proposal will inevitably be modified:
DONALD TRUMP: Under my proposal, it's the biggest tax cut by far, of any candidate by far. But I'm not under the illusion that that's going to pass. They're going to come to me. They're going to want to raise it for the rich. Frankly, they're going to want to raise it for the rich more than anybody else.
But the middle class has to be protected. The rich is probably going to end up paying more.
Another issue where Trump has been accused of trying to take two sides: the minimum wage.
After he seemed to express an openness to raising the minimum wage, Trump clarified, "I would like to see an increase of some magnitude. But I'd rather leave it to the states. Let the states decide."
Empathetically, he added: "I don't know how you live on $7.25 an hour."
On his previous support for Hillary Clinton, Trump admitted that when he was solely a businessman he wasn't "looking to get in fights with politicians."
However, Trump explained, "The fact is, she has not done a good job. When I look at what happened with Libya ... with Benghazi."
Yet Trump was sure to be careful with his public comments. "I'm not looking to get into wars with politicians, because I need politicians," he said.
"You don't learn much from tax returns"
Finally, on releasing his tax returns, Trump claimed, "You don't learn much from tax returns," yet even so, "I would love to give the tax returns. But I can't do it until I'm finished with the audit.
Trump pledged to release his tax returns before the election, so long as "the auditors finish."