With perfect timing, John McCain has signaled the opening of the vice presidential speculation season, just as the last dregs of suspense are being drained from the marathon presidential contest. By inviting three of the hot prospects for his running mate to his Sedona, Ariz., ranch for a weekend barbecue-cum-audition, the Republican standard-bearer has given official sanction for journalists and politicians to start calculating who will occupy the second lines on all the ballots come November. Conventional wisdom is that the winners of this guessing game hold only the briefest of interest to voters. Until the night of the traditional...