Lagging far behind in the crucial delegate count to win the Republican presidential nomination, Sen. Marco Rubio (R-Fla.) seemed to concede on Wednesday that his decision to go after Donald Trump in personal terms hurt his prospects.

The Florida Republican insisted that he felt a responsibility to stand up to Trump, arguing that the businessman had bullied too many minority groups, journalists and fellow candidates to not be called out. But, he added, he wouldn't have mocked Trump on such trivial matters as his penis size, his face and his tan, if he had to do it again.

"My kids were embarrassed by it," Rubio told NBC's Chuck Todd. "If I had to do it again, I wouldn't."

"I think he had to be stood up to. I really do believe that. But that said, that's not the campaign I want to run," he said.

"If that's what it takes to be president of the United States, then I don't want to be president."

Lest Todd think Rubio was gunning for a vice-presidential nod, he put that option to rest. The senator said he wasn't interested, and definitely not game if the person offering him the position is Trump.

"I'm not going to be the vice president," he said. Pressed if he would say no if Trump asked, he replied: "Absolutely."

Conservative pundits had loudly cheered Rubio's decision to take on Trump both at the debates and on the trail, making the argument that no other candidate had launched a sustained assault on the front-runner, which could chip away at Trump's lead.

But the assault was born of desperation -- Trump was threatening to run away with the nomination -- not strategic calculation. And it was clear from the get-go that Rubio was risking his own positive approval ratings by getting into the proverbial mud.

Sure enough, there is no evidence to suggest that the attacks have helped Rubio. In fact, they seem to have hurt. Rubio suffered a terrible series of primaries since then and now finds himself in a seemingly impossible hole with rumors circulating that he might leave the race even before his home state of Florida votes Tuesday.