Vice-presidents: Behind every good candidate...

Lifting the 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.

Media outlets had reported that the 41-year-old Mr Rubio had not made the expected Republican nominee's vice-presidential shortlist. But during a campaign stop in Michigan, Mr Romney said the story was "entirely false" and 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 with 29 electoral college votes of the 270 needed to win.

Whether he will be on Mr Romney's final shortlist is less clear. The winnowing began more than two months ago, the moment Rick Santorum, his last credible rival for the nomination, dropped out of the race, and speculation has been feverish ever since.

Other possibilities include the Ohio Senator Rob Portman, the former Minnesota governor Tim Pawlenty, the rising Republican Congressman Paul Ryan, and New Jersey's bluntly spoken governor, Chris Christie.

But speculation is all it is. The Romney campaign prides itself on discipline and ability to keep a secret, and the candidate told reporters on Tuesday that, apart from him, the only person who knows who is on the list is his trusted aide Beth Myers, his chief of staff when he was governor of Massachusetts, who is handling the vetting.

But there have been some clues to Mr Romney's thinking. The biggest of these is his repeated insistence that his running mate will be someone who is ready and qualified to take over as president if necessary. In other words, Mr Romney will bend over backwards not to repeat the mistake made by John McCain, the last Republican nominee, who selected the flashy but woefully unqualified Sarah Palin.

The choice cast serious doubt on the judgement of Mr McCain (who, had he won in 2008, would at 72 have been the oldest man ever to become president) and almost certainly contributed to the size of his defeat.

Under the constitution, a vice-president's formal duties are few – basically they come down to presiding over the Senate, where he casts the deciding vote in the event of a tie – and many have chafed at the limitations. Famously, John Nance Garner, Franklin Roosevelt's vice-president from 1933 to 1941, described the office as "not worth a bucket of warm piss."

In practice, a vice-president's job and influence (or lack of them) are determined by the president himself. Lyndon Johnson, for instance, was virtually excluded from John Kennedy's inner circle. By contrast, under the inexperienced George W Bush, Dick Cheney ran what amounted to a parallel administration, especially during Mr Bush's first term.

A vice-president's biggest responsibility is the unspoken one: the possibility, far from theoretical, 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 the death or resignation of their boss – most recently Gerald Ford after the fall of Richard Nixon in 1974.

By this yardstick, Mr Rubio, the son of Cuban immigrants, may be found wanting. Despite his undoubted charisma, he has been a US senator for barely 18 months, with just eight years in Florida's state legislature before that. That lack of experience could also rule out both Mr Christie and Bob McDonnell, governor of Virginia, both of whom only took office in 2010.

With Jeb Bush, the former Florida governor, having taken himself out of consideration, the most obvious front-runners are Mr Portman, a senator and former congressman who served as George W Bush's budget director, and Mr Pawlenty, a two-term governor of the swing state Minnesota, who has a popular touch that Mr Romney lacks.

But nothing is certain, neither the identity of Mr Romney's running mate, nor the date of an announcement. Most often, the choice is made on the eve of the convention – this year in late August. But Mr Romney, who is cautious and well organised, may act sooner, perhaps as early as next month.

Vice squad: Those who made a difference, for better or worse

Lyndon Johnson:

The Texan LBJ may have been the last vice-presidential nominee who helped the ticket in terms of electoral votes, shoring up the wobbling Democratic south for John Kennedy in 1960. Ironically, Johnson was among the most ineffectual occupants of the office, and hated the job.

Spiro Agnew:

He brought little to the Republican ticket in 1968. But in 1973 he became only the second vice-president in history to resign, after admitting taking bribes when he was governor of Maryland. Agnew's fate prefigured that of his boss Richard Nixon nine months later.

Sarah Palin:

For a few days, John McCain's choice of Palin, then governor of Alaska, seemed inspired. But her pitiful lack of qualification soon became clear. The notion of Palin sitting the proverbial "heartbeat from the presidency" undoubtedly was a factor in his defeat.

Dick Cheney:

Cheney, from rock-ribbed Republican Wyoming with just three electoral votes, was of little electoral help to George W. Bush in 2000. But he went on to become arguably the most powerful and most divisive vice-president in US history

Al Gore:

The selection of Gore in 1992 was a groundbreaker, as Bill Clinton ignored traditional criteria of generational and geographic balance and picked a young southern moderate like himself, on a "Double Bubba" ticket. Gore was also first "modern" – i.e. more consequential – vice-president.

Dan Quayle:

Astonishment greeted George H.W. Bush's pick of the young Indiana Senator as his running mate in 1988. As vice-president Quayle became a figure of fun (famously even misspelling the word "potato" in a school class.) His gaffes may have marginally contributed to Bush's loss in 1992.