There have been a number of instances in recent history where the choice of a vice presidential running mate was an important stepping stone toward winning in the fall.
Of course, it's much too early to know how much of a difference GOP presidential candidate Mitt Romney's choice of Wisconsin Rep. Paul Ryan will make. In the meantime, here is my subjective list of the top five instances in the past half-century or so where a selection of a running mate was crucial to victory:
1. 1960: John Kennedy-Lyndon Johnson (D)
Without a doubt, No. 1. Most analysts argue that in naming Lyndon Johnson, his chief rival, as his VP in 1960, John Kennedy went on to carry a region of the country that he may not have gotten with a non-Southern running mate — a decision that gave Kennedy a huge electoral college victory that year, even if the popular vote was incredibly close. They didn't particularly like each other, having run against each other for the nomination, and we know all about Johnson's relationship with Robert Kennedy. It was an enmity that perhaps lasted all the way until Dallas. It was the first time a presidential ticket was composed of two senators. And in picking Johnson, JFK followed a familiar path of his party's standard-bearer in looking South to a running mate (see: John Nance Garner, Alben Barkley, John Sparkman, Estes Kefauver, etc.). But LBJ was not just a Southerner; he was the most powerful member of the Senate and gave the ticket some conservative credentials. His unhappiness in the job for the next three years is another story.
2. 2000: George W. Bush-Dick Cheney (R)
The selection of Dick Cheney brought along congressional, Cabinet and foreign policy experience — everything that George W. Bush lacked. He also brought along Washington know-how. It's debatable how much Cheney helped during the campaign (a campaign where Al Gore actually won the popular vote), but once in office Cheney proved to be in an invaluable and influential — if controversial — vice president.
3. 1976: Jimmy Carter-Walter Mondale (D)
If George W. Bush had few D.C. credentials, it was even worse for Jimmy Carter. Hardly the choice of the Washington establishment, Carter, a one-term governor of Georgia, actually fed off the public's antagonism towards Washington in this Watergate-inspired election. But he still needed to be able to work with the folks on Capitol Hill, and in picking the widely respected Walter Mondale as his running mate, he got someone who came to the table with 10 years' experience in the Senate.
4. 1980: Ronald Reagan-George H.W. Bush (R)
By selecting George H.W. Bush, his chief rival for the nomination, Ronald Reagan sent a message that the party needed to be united if they were going to defeat President Carter. But it was more than that. Almost in response to the charge that he was too out of the mainstream to win, Reagan picked Bush, who as a former congressman, UN ambassador, RNC chair and CIA director, had solid moderate and establishment credentials.
5. 1992: Bill Clinton-Al Gore (D)
All the so-called rules of "ticket balance" were thrown out when Bill Clinton picked Tennessee Sen. Al Gore to run with him. There was no regional, ideological or demographic balance. Both were young (Clinton 45, Gore 44) and both professed centrist positions on many issues. Coming from neighboring states, they were the Democrats' first all-Southern ticket since 1828. But there was an obvious comfort level between the two, and Clinton's intention to redefine his party succeeded.
Honorable mention: Barack Obama selecting Delaware Sen. Joe Biden in 2008. Obama had been in the Senate for only 15 minutes, and his pick of Biden, in office since 1973, helped balance the experience gap. And Biden fared well in comparison with the clearly over-her-head GOP choice of Sarah Palin for vice president.
All six examples described above were victorious. I'm hard pressed to pick a good running mate selection for a losing ticket in the past half-century. Most of them made little or no difference at all. I'd have to go back to 1948, when the Republican presidential nominee, New York Gov. Thomas Dewey, chose California Gov. Earl Warren as his No. 2. Warren was exceptionally popular at home. And for all the old ticket-balancing reasons, Warren made sense. California was a state of rising importance back in '48, and while it had only 25 electoral votes (compared to 55 now), it trailed only New York, Pennsylvania and Illinois in that category. Still, Warren failed to help the GOP ticket carry the Golden State.
And the most disastrous VP pick? That has to be George McGovern's selection of Sen. Thomas Eagleton of Missouri in 1972. Back in those days, when vetting was hardly an art, nobody knew about Eagleton's electric shock treatment for depression, a fact that came out after the Democratic ticket was announced at the Miami Beach convention. Eagleton was gone in just over two weeks, replaced by Sargent Shriver.