An ad posted on Twitter mocking Florida Sen. Marco Rubio as "Little Rube" was nothing personal, says GOP presidential front-runner Donald Trump, just payback for Rubio's attacks on him.The ad, posted on Trump's Twitter account on Sunday, shows a picture of Rubio as an elementary school student contrasted to a recent photo of the billionaire businessman Trump appearing to adjust his cufflinks."Never Hire a Boy to Do a Man's Job," the ad is headlined."Marco Rubio seems like a nice boy," the copy begins. "Well mannered, but bought & paid for. JEB and the PACS sat him down and taught him all that fancy government stuff. 'RINO RUBE' repeats it well! Good boy!""Little 'RUBE'" doesn't have the "swagger" to negotiate with Russian President Vladimir Putin, the ad says, suggesting instead a "real man" like Trump will handle things until Rubio is "all grown up."Trump appeared Monday on Fox News Channel'ssaying Rubio is "just fine," but argued that Rubio hit him first.Rubio initially said he wouldn't fall for Trump's bait, buthas called Trump "touchy" and "insecure" and unable to "have more than a 10-second sound bite on any key issue."He appeared to be baiting Trump himself when he said he had no interest in participating in Trump's "freak show."Trump freely admits to living by the code that he won't attack anyone unless first attacked – and then it appears to be mandatory for him to strike back.His ad did just that, pointing out the question that many voters have of Rubio: Is he too young?Rubio is 44, but jokes to crowds that he "feels 45."In an interview aired on Fox News Channel'sMonday, correspondent John Roberts told Rubio, "You are one of the youngest looking 44s I've ever seen.""That's how God made me," the fresh-faced Rubio smiled. "I have no problem with it."Trump told Van Susteren that Rubio attacked him because he is low in the polls.Rubio has actually been climbing recently and is challenging the three outsiders, Trump, neurosurgeon Ben Carson and businesswoman Carly Fiorina.That could be another reason Trump sees a need to take him down a peg."I told people that he is weak on immigration — very, very weak on immigration, actually, which is a very sad thing, Trump said. "And I think that's one of the many things that people are liking my about my candidacy, that I'm strong on stopping illegal immigration."And the other thing is his voting record is horrible," Trump said. "If you look at his voting record in the Senate, he misses so many votes – much more than anybody else. I brought out these things. But I have nothing against him personally at all."