I don’t think liberals will even be the problem with Rubio. Much like Clinton first raised concern’s about Obama’s eligibility, I’m sure one of his primary opponents will raise concerns about Rubio’s eligibility.
However, the challenge will go nowhere. Rubio was born in the United States. There’s no case law supporting any of these notions about “natural born citizen” and a court isn’t going to venture into this territory in 2013.
If the republicans were smart, they would not allow someone of questionable legality to run. But they don’t call it the “stupid party” for nothing. It does not matter what a party rival may do to Rubio, the democrats know how to effectively use simple mistakes against republicans. If Rubio runs, it will be a mistake. Through their friends in the MSM, the dems will make sure the entire country knows it.