Four years ago it was all about Ohio. Four years before that the election hinged on Florida. But on Tuesday night, watch Pennsylvania.
Here's why: right now Barack Obama is ahead in a range of states that were won by Republicans last time around, including Virginia and Ohio.
He's also leading McCain is states like Colorado by six points, in Nevada by seven points, and North Carolina by six points.
The only way McCain can find a viable path to 270 electoral votes is if he steals Pennsylvania's 21 electoral votes away from the Democrats.
But it's not going to be easy. It's a state that hasn't gone for a Republican presidential candidate since George H.W. Bush in 1988.
And Obama is leading McCain in Pennsylvania by double-digits, according to some state polls.
Still, McCain is hoping he can sway Independents and working-class voters there, many who went for Sen. Hillary Clinton in the primaries.
That's why he has spent so much time there. That's why the McCain campaign has lavished so much money there. It's why they keep coming back to the state again and again.
It's McCain's must-win state Tuesday night.