Earlier in the day, Romney had declined to comment on reports that Rubio, a rising star in Republican politics, was not under consideration as a potential running mate.
The presumptive GOP nominee initially told Fox News only that "a number of people are being vetted" but that only two people - he and a senior adviser - know who's on the list. He repeated that statement Tuesday evening but clarified Rubio's status as a potential vice presidential pick.
The statement was an unusual departure from the secrecy that has surrounded Romney's process in selecting a running mate. But it speaks, in part, to Rubio's political influence among the Republican base and Hispanic voters.
Two Romney representatives would not say if or when Rubio had submitted paperwork for the vetting process.
The flap came on the day Rubio released a memoir, American Son, and Romney concluded a six-state bus tour in Michigan, the state where he was born. On Wednesday evening, Rubio will discuss the book at the National Constitution Center in Philadelphia.
Asked about the vetting reports during an appearance on Fox News, Rubio declined to weigh in.
"I'm not commenting on the vice presidential process," he said. "That's been basically what we've said the whole time because, out of respect for Gov. Romney, the last thing he needs is to have to be addressing questions about this, because really the campaign's not about that."
A handful of possible picks joined Romney on his bus tour in recent days as part of unofficial public tryouts for the No. 2 spot. Their interactions offered clues about who Romney might choose.
Former Minnesota Gov. Tim Pawlenty traveled on Romney's bus with him for two full days, on Friday in New Hampshire and through Saturday in Pennsylvania.
On Sunday, Ohio Sen. Rob Portman and his wife, Jane, went along for the ride.
Less visible was Wisconsin Rep. Paul Ryan, whose role was limited to introducing Republican National Committee Chairman Reince Priebus at an event at a factory in Ryan's hometown of Janesville. Wisconsin Gov. Scott Walker, whose June 5 recall-election victory was a big win for Republicans, introduced Romney and accompanied him on a tour through the factory.