When it comes to vice presidential nominations, the scholars, journalists, and politicians who know and care the most about the subject know something else as well: namely, that people vote for president and not vice president. This latter knowledge is inconvenient. If it doesn’t really matter who the nominees for vice president are, then how can we justify all the time we spend obsessing over who John McCain and Barack Obama will choose as their running mates ?

I recently spoke to this question at a wonderful gathering of political consultants, reporters, and election scholars organized by the indefatigable John Geer at Vanderbilt University. The same iconoclasm that animated John’s prize-winning book, In Defense of Negativity: Attack Ads in Presidential Campaigns (Chicago, 2006) spurred in him the original thought that these folks actually had something to learn from each other.

I stated the theme of my brief remarks at the outset: I’m here to enable you in your guilty pleasure. For at least five good reason, it really does matter who the candidates for vice president are.

First, there is a very good chance that the next vice president will become president. Historically, nine vice presidents have succeeded to the presidency and another five have been elected. On average, about one in three vice presidents is a future president. And that’s not counting those who, like Richard Nixon in 1960, Hubert H. Humphrey in 1968, and Al Gore in 2000, came very close.

Second, between now and election day, the vice presidential candidates are likely to dominate the media’s coverage of the campaign for at least three weeks: the weeks that the two are chosen and the week of their debate. If one (or both) of the candidates is exceptionally weak — think Spiro T. Agnew in 1968, Thomas Eagleton in 1972, Geraldine Ferraro in 1984, and Dan Quayle in 1988 — their time in the national spotlight will be even longer.

Third, even if 98-99 percent of the voters ignore the vice presidential candidates when they cast their ballot, the remaining 1-2 percent constitute more than one or two million people — enough to tip most close elections. How many elections are close? During the last half century, the elections of 1960, 1968, 1976, 2000, and 2004 have been — that’s 5 out of 12, or nearly half.

Fourth, it’s a cliche and it’s true: choosing the running mate is the first presidential decision that candidates for president make and they can be fairly evaluated on that basis. One thing I always look at: Is the wannabe president secure enough in his own self-confidence to choose someone who is at least as impressive as he is?

Finally, the days when the vice presidency wasn’t “worth a pitcher of warm spit” (the G-rated version of FDR vice president John Nance Garner’s famous aphorism) are long gone. For well over a generation, the vice presidency has been an office of real prominence and influence.

So live it up, political junkies. All those hours you’re spending watching Chris Matthews and Britt Hume or buzzing over coffee with your fellow political junkies — they’re hours well spent.

Now for our contest. The prize is a free copy of The Elections of 2008, forthcoming from CQ Press early next year. Answer these questions by using the comment box below:

1. On what date will John McCain announce his choice? 2. On what date will Barack Obama announce his choice? 3. Will either choose someone who is not a white male? If so, will it be McCain, Obama, or both? 4. Will either choose a governor or other non-Washington figure? If so, will it be McCain, Obama, or both? 5. How much younger than McCain will McCain’s running mate be? 6. How much older than Obama will Obama’s running mate be?