— 2. A vice presidential nominee's most important role is to balance the ticket.
This is true sometimes, as when a young and relatively inexperienced Barack Obama picked 36-year Senate veteran Joe Biden in 2008, or when Washington insider Dick Cheney suggested himself to George W. Bush in 2000 to counterbalance Bush's perceived lack of gravitas and foreign policy experience.
But one of the most successful contemporary political pairings was of two wonkish, 40-something, white Southerners: Clinton and Al Gore in 1992. Gore's selection reinforced the message of generational change that Clinton wanted to send as he unseated the last president of the World War II generation, George H.W. Bush.
Clinton and Gore also reported that they had great "chemistry" and enjoyed campaigning together, qualities said to also be high on Romney's list. But will he find his political soul mate in someone comfortably familiar, such as Sen. Rob Portman of Ohio, or someone younger and ideologically edgier, such as Rep. Paul Ryan of Wisconsin?
3. A vice presidential nominee can carry a key swing state.
This has not happened since 1960, when Lyndon Johnson, ahem, helped John Kennedy win Texas. But that was a time when political machines (or chicanery, in the case of the 1960 voting fraud allegations) could still have a major impact on turnout.
Since then, presidential nominees have generally ignored this consideration, or, if they have tried to heed it, it has not worked. Massachusetts Gov. Michael Dukakis attempted to revive the Kennedy-Johnson "Boston-Austin Axis" in 1988, but having Sen. Lloyd Bentsen on the ticket could not make Texas a Democratic state again. Nor could Edwards deliver his native North Carolina for John Kerry in 2004.
The near-abandonment of this strategy has been a boon to vice presidential nominees from less populous states, such as Cheney of Wyoming and Palin of Alaska. Sen. John Thune of South Dakota should take heart.