Strategists say a loss in Pennsylvania would all but doom the president’s reelection hopes. It would mean he hadn’t rallied his base, or won back independent voters who abandoned him in 2010, or closed an enthusiasm gap that now favors Republicans. A poor showing here — a state the Democratic nominee has carried in the last five presidential contests — would suggest Obama’s surprising 2008 victories in states such as Virginia and North Carolina would be tough to duplicate.

It would not bode well for Obama in states where he faces even tougher fights, such as Ohio and Indiana, which share Pennsylvania’s post-industrial hardships. Even a close race in Pennsylvania would suck up Obama’s time, keeping him out of Southwestern states such as Nevada, New Mexico and Colorado that offer another route to amass the 270 electoral votes needed to win.

“There’s a theoretical path to 270 without Pennsylvania, but not a practical path,” said Steve Schmidt, who ran Republican Sen. John McCain’s 2008 campaign. “You can manipulate the states and the numbers to arrive at 270, but losing Pennsylvania would indicate deep problems” in the Obama campaign.