Pennsylvania’s Democratic primary, to be held April 22, is the largest single delegate haul remaining for Barack Obama and Hillary Clinton. While its outcome is unlikely to immediately end the contest, its results may determine how long the race will last.

Clinton, enjoying the support of popular governor Ed Rendell and Philadelphia mayor Michael Nutter, plays well to middle- and working-class whites with strong labor connections here. She has spent the better part of a month stumping in the state and hopes to foreground her experience on the economy (noting disagreements with her husband by opposing continental free trade) to distance herself from Obama.

Though Clinton has held double-digit leads in most polls for the last month, recent numbers suggest that Obama is closing the gap. Obama’s campaign is hoping to “exceed expectations” in the state – political speak for hedging their bets in what amounts to Clinton’s backyard.

If Obama can narrow the margin to three to five points on primary day, he can continue his claims that he has made strong inroads into Clinton’s core constituencies and could carry them in a general election.