Marco RubioMarco RubioGOP loses top Senate contenders How does placing sanctions on Russia help America? Republicans play clean up on Trump's foreign policy MORE regrets his personal attacks against his GOP competitor for the White House Donald TrumpDonald TrumpEx-CIA analyst resigns rather than serve Trump administration Matt Schlapp op-ed: Challenges, controversy won't stop CPAC 2017 Anti-Trump protests swell outside Parliament during debate on official visit MORE, he told MSNBC Wednesday. 

"In terms of things that have to do with personal stuff, yeah, at the end of the day it's not something I'm entirely proud of," Rubio told the network in a town hall on Wednesday night. 
 
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"My kids were embarrassed by it, and if I had to do it again I wouldn't." 

Rubio had previously shied away from direct confrontation with Trump, but he burst out of the gate in the last debate before Super Tuesday by emptying the opposition research file against Trump.
 
The next morning, he bashed the billionaire as a "con artist" on television and days later poked fun at Trump's hands for being small.

"You know what they say about men with small hands?" Rubio said at a Feb. 26 rally. "You can’t trust them.” 

Despite the more aggressive tack, the Florida senator hasn't gained traction in the GOP nomination fight, falling hundreds of delegates behind Trump and Ted CruzTed CruzTrump to speak at CPAC Trump to interview four candidates for national security adviser Milo Yiannopoulos to speak at CPAC MORE. His campaign has pinned all of its hopes on next week's Florida primary. 
 
At the same time,he told host Chuck Todd, "he needed to be stood up to."
 
Rubio also told MSNBC that he wouldn't be anyone's running mate -- including Trump's -- if asked. "I'm running for president."