Mitt Romney may turn to Rubio as his number two

Lifting a rare veil on one of American politics' most hermetic procedures, Mitt Romney has declared that, contrary to some speculation, he is indeed considering the popular Florida senator, Marco Rubio, as a possible running mate in November.

The unexpected revelation came after the media reported that the 41-year-old Mr Rubio had not made it onto the expected Republican nominee's vice presidential shortlist. During a campaign stop in Michigan on Tuesday, Mr Romney described the story as "entirely false", stressing that "Marco Rubio is being thoroughly vetted as part of our process".

That Mr Rubio has been under consideration is no secret. He is a hero of conservatives and the Tea Party movement, constituencies that have been wary of Mr Romney in the past.

A Hispanic from Florida, he might help sway an ever more important voting group put off by the candidate's hardline views on immigration. Then there is the small matter of Mr Rubio's home state, a key prize in November.

But whether he is on Mr Romney's final shortlist is less clear. The winnowing effectively began more than two months ago, the moment Rick Santorum, his last credible rival for the nomination, dropped out of the race. Among other names often mentioned are those of Ohio Senator Rob Portman, former Minnesota governor Tim Pawlenty, the rising Republican Congressman Paul Ryan, and New Jersey's blunt spoken governor Chris Christie.

But speculation is all it is. The candidate told reporters on Tuesday that only two people – himself and trusted aide Beth Myers, his chief of staff when governor of Massachusetts and who is handling the vetting – know who is on the list.

There have, however, been some clues. The biggest is Mr Romney's insistence that his running mate will be someone qualified to take over as president if necessary. In other words, he doesn't want to repeat the mistake made by John McCain, the last Republican nominee, who selected the woeful Sarah Palin.

Under the constitution, a vice-president's formal duties are few – basically to preside over the Senate where he casts the deciding vote in the event of a tie – and many have chafed at the limitations.

The biggest responsibility is the unspoken one: the possibility that he might have to step into the top job at a moment's notice. Nine of the country's 44 presidents have been sitting vice-presidents catapulted into the Oval Office by death of resignation of their boss – most recently Gerald Ford after the fall of Richard Nixon in 1974. By this yardstick, the 41-year-old Mr Rubio may be found wanting. Despite his charisma, he has been a US senator for barely 18 months. That lack of experience could also rule out both Mr Christie and Bob McDonnell, governor of Virginia. The most obvious frontrunners are Mr Portman, senator and former Congressman who served as George W Bush's budget director, and Mr Pawlenty, governor of Minnesota, who has a popular touch Mr Romney lacks.

The choice is made on the eve of the convention, this year in late August, but Mr Romney may act sooner.