Mitt Romney is heading to Pennsylvania to campaign this weekend, the most visible evidence to date that his campaign thinks it has a shot at winning the reliably blue state. In addition, the Romney campaign and its affiliated super PACs are flooding Pennsylvania's airwaves with new advertisements, forcing President Obama and his allies to play defense with new ads of their own. GOP candidates have long made noise about competing in the Keystone State, but Pennsylvania has voted for the Democratic candidate in the last five presidential elections (the last GOP winner: George H.W. Bush in 1988). Obama currently holds a nearly 5-point lead in the polls, according to the Real Clear Politics average. So why does Romney think he can pull off an upset?

1. The state is ripe for advertising
"If ever there were a place where a last-ditch torrent of money could move the needle," it's Pennsylvania, says Jeremy W. Peters at The New York Times. With the state long presumed as being in Obama's column, there has "been a void of presidential ads." Indeed, Obama's durable lead in the all-important swing state of Ohio is partly due to the fact that it was "carpet-bombed with negative ads" about Romney, says Jennifer Rubin at The Washington Post. "Pennsylvania was relatively free from the onslaught," giving Romney "fertile ground to plow."

2. Obama's support is slipping
Obama's lead in Pennsylvania is only in the single digits, a significant drop from 2008, when he beat John McCain by 11 points. And remember, Pennsylvania is home to "a large number of white working class voters," says Steve Kornacki at Salon, "a group that Obama has struggled with across the country during his presidency." In addition, Romney may be making gains with "upper-middle-class women and Jewish voters" who are socially moderate but unhappy with the direction of the economy, says Peters. "Among recent ads is one in which a woman directly refutes Obama ads that portray Mr. Romney as extreme on reproductive health issues."

3. Romney has no choice
"It's difficult not to compare to this move to an on-side kick in football," says NBC's First Read, "when you're behind by a touchdown with a few ticks on the clock left." With Obama stubbornly holding on to his slim lead in Ohio, Romney has no choice but to explore options outside the traditional swing states. The fact that he's "scrambling to compete in Pennsylvania, Michigan, and Minnesota is, at this point, a sign of desperation, not confidence," says Kornacki. "It's baffling why the Romney campaign left these states until the last minute," given how tough his electoral path to victory would be without Ohio.