The McCain strategy depends on holding a handful of Bush '04 states that are now rated tossups by CNN: Florida, Ohio, North Carolina, Missouri and Nevada. It also depends on keeping Virginia, which CNN now considers leaning Democratic, in the GOP column.
But even if McCain won all six of those states, in addition to those in which he is already favored, he would still be shy of the 270 electoral votes needed to claim the presidency.
Which is where Pennsylvania and its 21 electoral votes come into play. Most polls show McCain trailing by 10 points or more in the state, but one top adviser told CNN on Monday, "The election hinges on Pennsylvania. We'll win Virginia and Nevada in the end, but lose without Pennsylvania."