In the history of the U.S. Presidency since 1865, on seven occasions the sitting president was unable to finish his term. All but one of those were due to death (Richard Nixon's 1974 resignation). And it should be pointed out that another of those seven, Franklin Roosevelt, died in office but in the beginning of his fourth term, which is a Constitutional impossibility these days.
So it's not all that common, of course, when the vice president has to step in and assume the reins of the Commander-in-Chief, mid-term.
Yet there is a fascination with the choice of presidential running mates every four years.
The vice presidency is a funny thing. You're, as they say, a "heartbeat away" from becoming the most powerful man in the world, yet while you're waiting for that to happen, you're as relevant as, well the vice president.
The job is, as former VP John Nance Garner once famously said, "not worth a bucket of warm piss" (yes, he said piss---not spit as has been mostly reported).
But we wait on pins and needles every four summers to see who the non-incumbent party's candidate will choose as his running mate. Yet on Election Day, we don't vote for vice president. No one goes into the voting booth pulling the lever for a man's running mate.
That doesn't stop the analysis or the hand-wringing or the speculation or the talking points, all about a decision that doesn't really have any bearing on the direction of the country.
The candidate usually picks the opposite of himself. If he's a loudmouth, he'll tab a quiet guy. If he's Northern, he'll go Southern. If he's right or left of center, he'll go with someone more central.
He doesn't even have to like the man he's choosing.
In 1960, Massachusetts' John Kennedy, who wrested the nomination from the likes of Texan Lyndon Johnson, felt he needed Johnson's appeal to Southerners and those who weren't crazy about JFK's being Roman Catholic.
So despite not being enamored with each other, the two joined forces on the Democratic ticket for the good of the party. It worked; Kennedy narrowly beat Richard Nixon on Election Day.
Does Paul Ryan matter? History says, not likely
The Kennedy/Johnson thing, though, is an exception to the rule. A presidential candidate's running mate---and indeed, a president's vice president---is mostly there not to embarrass his (or her) boss. Hence the occasional hoof-in-mouth Joe Biden, who sometimes makes Barack Obama cringe, no doubt.
Even dunderheads like Dan Quayle didn't keep his candidate from winning.
Did Sarah Palin prevent John McCain from beating Obama in 2008? Well, she likely didn't help, although I assure you the race was between McCain and Obama, not Palin and Biden.
Paul Ryan is Mitt Romney's choice for vice president. Immediately after being picked, Ryan's record on Medicare became the star of the day's 24-hour news cycle. Polls show that, several days after being selected, Ryan hasn't changed the Obama/Romney numbers all that much, if at all.
Which is how it should be. Which is how it isn't treated, until Election Day, when voters vote for president, not vice president.
John Nance Garner knew that better than anyone.