Donald Trump adviser Gen. Michael Flynn lashed out at Mark Cuban on Sunday, declaring that the anti-Trump billionaire should not be allowed to attend Monday night's presidential debate because he is "not a legitimate person."

In response the the news that Cuban will attend the first presidential debate of the 2016 general election, Trump tweeted over the weekend that he may invite Gennifer Flowers, who allegedly had an affair with Hillary Clinton's husband decades ago.

On Sunday, Flynn deflected questions from NBC's Chuck Todd about the invitation to Flowers.

"I would just go with what you have seen," Flynn said. "And we'll wait to see what happens tomorrow night."

"Was it appropriate to invite Mark Cuban?" the Trump adviser continued. "I mean, he's not a legitimate person. Why is he invited? I mean, again, I would leave this tit for tat. This is about the big issues this country is worried about."