John Nance "Cactus Jack" Garner, who reluctantly agreed to be Franklin D. Roosevelt's running mate, pungently described the vice presidency as being "not worth a bucket of warm spit," except that he probably used a vulgar term for another bodily fluid.

Well, we've come a long way since then. Last week, the popular favorite for this year's Republican vice presidential nomination, Sen. Marco Rubio of Florida, came all the way up to Delaware County to campaign with Mitt Romney before a Pennsylvania primary divested of immediate relevance.

Rubio has everything Romney seems to lack: charisma, the common touch, genuine humor, eloquence, and a record of consistent conservatism supplemented by an informed emphasis on foreign policy. Moreover, his joining the Republican ticket should energize Hispanic voters, including his fellow Cuban Americans, in the electorally important state he represents.

With polls putting Romney in a virtual tie with President Obama, might 2012 turn out to be one of those rare election years when a running mate could actually tip the balance? It could depend on whether the 40-year-old Rubio, now serving only his second year in the Senate, would view a departure as premature, leaving the "veepstakes" open to any number of other contenders.

Our Constitution leaves a lot about the vice presidency open to interpretation, giving the position few specific responsibilities. The vice president presides over the Senate, where he can break ties, and takes on more substantive tasks only at the discretion of the chief executive. These have expanded over the decades, from ceremonial and political duties to actual policy-making.

However, the vice president's most significant function has always been one we hope will be unnecessary: to serve as the president's understudy.

Alas, our history has not been serene. On two occasions, we have had three different presidents in a single year. Four presidents have been assassinated, and twice as many have been the target of unsuccessful attempts. Others died in office or were incapacitated. And one was obliged to resign.

It was once generally understood that in any such event, the vice president would serve as "acting president" until a new election could be held. That wasn't good enough for stubborn John Tyler. He insisted on serving out the full term of his also headstrong predecessor, William Henry Harrison. Only 126 years later, the 25th Amendment codified Tyler's view of presidential succession.

Last week, about when Sen. Rubio was rousing Pennsylvania crowds for Mitt Romney, former Vice President Dick Cheney stressed to a youthful audience in Washington that the most incomparably important consideration in selecting a vice presidential candidate is whether he or she is fully qualified and prepared to ascend to the presidency. That implies more extensive vetting than Sarah Palin withstood in 2008, although she surely added zest to the ticket.

Speaking of 2008, Obama has never ceased to maintain that his "first and best" decision as a candidate was to name Joe Biden his running mate. They do seem to have a good time together.

It's commonly concluded that "nobody votes for vice president." But many prospective vice presidents have outperformed the top of the ticket, and there have been occasions when they made the difference. "Landslide" Lyndon Johnson's efforts secured John F. Kennedy's razor-thin margin in 1960. And though you won't read about it in most history books, the innovative "prop-stop" airborne campaign of the man who became the first "veep," Alben Barkley, was a crucial supplement to Harry S. Truman's celebrated "whistle-stop" train tour of 1948. Without it, they might not have upset the GOP "dream team" of New York's Tom Dewey and California's Earl Warren.

The latter pair's problem was that they profoundly disliked each other, while the Democrats' border-state duo worked well together — which is really the most important consideration of all. In politics, as in music, harmony is what makes everything possible.

Harold I. Gullan is a Philadelphia historian and the author of "Toomey's Triumph."