(CBS News) WASHINGTON - Mitt Romney denied reports Tuesday that Florida Sen. Marco Rubio is not being considered as a possible running mate. Sources tell CBS News, however, that Rubio is unlikely to be Romney's choice in the end. It's an issue the presumptive Republican presidential candidate would rather not talk about. A vice presidential pick is the first big decision voters will see from the presidential nominee, and in a close race, that choice could make a difference. Because it can be so important, the contenders are always kept secret. Presidential campaigns don't talk about who's on their short list -- until they feel they have to. And that's where Romney found himself Tuesday. Rubio mum on VP process Dickerson on GOP veepstakes Throughout the day, the story picked up steam: Rubio reportedly wasn't being vetted as a possible running mate. A favorite of conservatives -- and a fundraising force -- Rubio deflected questions. He told Fox News, "I'm not commenting on the vice presidential process. ... I just don't talk about the process at all. Finally, Romney had had enough. Late Tuesday, he broke his silence to say Rubio was, in fact, on his short list. "This story was entirely false," Romney said. Romney also had a note of caution for reporters: Those who talk don't know, and those who know don't talk. There are only two people in this country who know who are being vetted and who are not. And that's Beth Meyers and myself. Myers is Romney's former chief of staff and is running the vice presidential selection process. "I know Beth well," Romney said. But there are clues. On his five-day bus tour, Romney campaigned with several possible contenders. In New Hampshire, he scooped ice cream with Sen. Kelly Ayotte and former Minn. Gov. Tim Pawlenty. In Ohio, the Romneys served pancakes to hundreds, with Sen. Rob Portman and his wife pouring the syrup. And at a textile mill in Wisconsin, he got a warm welcome from budget hawk Rep. Paul Ryan. Then there are other prospects: South Dakota Sen. John Thune and Louisiana Gov. Bobby Jindal. They weren't on the bus tour, but have been on the campaign trail. In an interview on "CBS This Morning" Wednesday - the first since Romney's comments Tuesday -- Rubio again refused to comment on the vice presidential selection process. The Romney campaign tells CBS News that, as a first-term U.S. senator, Rubio's a long shot, and that if you're betting, you should probably put your money on Portman or Pawlenty.