Ideally, Obama would prefer a running mate who doesn’t show him up. He doesn’t want the Lloyd Bentsen effect. (Lloyd Bentsen was such a highly regarded VP pick that it begged the question why Dukakis was at the top of the ticket. Bentsen was picked to make Dan Quayle look small, which he did, but his age and experience also made Dukakis look small--figuratively and literally)
So, in terms of making Obama look good Tim Kaine is ideal. That presumes that the election is so lop-sided that one can seek to perfect a certain image of freshness.
In a closer election there’s no room for another unknown, and reassurance becomes key. (“I’m not sure about Obama, but at least he’ll have X around, who I know better.”)
And in a losing cause the VP selection is a chance to make a Hail Mary pass. (No woman had been on a ticket before 1984, and Mondale was so doomed that it was worth finding out whether women would turn out to vote for a woman. Nobody knew, so it was worth a shot.)
National polling
week of August 18 _________ Best VP pick
Obama +6 or more __________ Tim Kaine
Obama +5 to -3 ____________ Joe Biden
Obama –4 to -9 ____________ Hillary Clinton
Obama –10 or more _________ Something weird, like Chuck Hagel or Mike Bloomberg
When people say, “The VP never decides an election,” they’re not entirely correct. Kennedy almost surely wouldn’t have won without Johnson (the popular vote difference was less than 0.1%) "Despite the reservations Robert Kennedy had about Johnson's nomination, the move proved to be a masterstroke for his older brother. Johnson vigorously campaigned for JFK and was instrumental in helping the Democrats to carry several Southern states skeptical of Kennedy, especially Johnson's home state of Texas."
http://en.wikipedia.org/wiki/United_States_presidential_election,_1960#The_fall_campaignAnd the biggest VP success story, which people forget or deny, was 2000.
The reason Gore won Florida was Joe Lieberman. Period. When the state with the second largest Jewish population is a tie, and your running mate is the first Jew on a major party ticket… the question answers itself.
By the way, McCain would really like a Jewish running mate. If not Joementum, then maybe that Jewish republican congressman from Virginia whose name I forget. Obama will win the Jewish vote, but margins are important. McCain could do worse for himself than a Jewish running mate from a swing state.