Barring an act of providence—usually silent on voting day, presumably out of respect for the democratic process—Senator Clinton will win Tuesday's primary race in Pennsylvania. 

But for the dance to go on, Clinton must not only win, but win big.  By the end of the primary season, neither she nor Obama will have bagged enough elected delegates for a conclusive victory.  She can only win if she persuades the superdelegates that she's more electable in the general election than the handsome upstart from Illinois. 

Clinton's electability argument is based on her track record of beating Obama in big states (must-wins in the general election) and in states dominated by "Reagan Democrats"—working-class whites who swing right for their values, but left for their wallets.  Pennsylvania—manufacturing country, hard-hit by the transition to a service economy—is both. 

So the race is Clinton's to lose.  And she recognizes that a numerical win is not enough.  As her husband learned in the 1992 New Hampshire primary, the media awards the "win" not to those who beat opponents, but to those who beat expectations.  If Obama overturns media expectations and loses only by a narrow margin, he remains the on-the-rise heir apparent. 

So in these final hours, the battle is over expectations.  Obama sets the bar to an achievable height, saying he will consider the day a success if he keeps Clinton's lead to the single digits.  Clinton's spokesman raises it higher, claiming if the well-funded Obama can't win Pennsylvania, it shows "serious problems winning the large states and closing the deal with voters."  Whose expectations will carry the day?  Today will tell.  Clinton will win the voting game.  What's up for grabs—and what her continued candidacy depends on—is whether she'll win the perception game.