Pennsylvania, which has chosen a Republican in the presidential election only four times in the past 50 years, is suddenly swinging toward Mitt Romney.

Keystone State voters are chasing a national trend that has taken hold in recent weeks, the movement of support away from President Obama and toward his Republican rival.

The most recent slate of polling in Pennsylvania shows Romney has nearly erased the double-digit lead Obama had there and is now within a few percentage points of the president.

Neither Romney nor his running mate, Rep. Paul Ryan, R-Wis., has spent much time campaigning in Pennsylvania, which last backed a Republican presidential nominee in 1988. But Romney's game-changing performance in a trio of presidential debates sharply improved his standing there.

"The debates were the driving force," said Harrisburg-based GOP strategist Charles Gerow.

The source of Romney's rise is the increased support he's getting in the fickle suburbs of Philadelphia, Gerow said. Those suburbs have backed Republican candidates in the past but sided with Obama in 2008.

"The last debate, where Romney appeared strong and demonstrated his leadership qualities without saber rattling, was appealing to a lot of women in southeast Pennsylvania," Gerow said.

Republicans have built a significant ground game in Pennsylvania, and legions of Romney volunteers are knocking on doors and contacting voters by phone, said Terry Madonna, director of the Franklin & Marshall College Poll.

Madonna said that while the polls "have definitely narrowed" between Obama and Romney, a victory for Romney "is still a tough haul."

But the Romney campaign sees an opening, and it has the Obama campaign suddenly playing defense.

After months of silence, Romney and Obama ads are on the air in Pennsylvania.

Three Romney-backing political action committees are pouring millions of dollars into ad buys that will fill the Pennsylvania air waves in the coming days.

Romney political director Rich Beeson, telling supporters he was encouraged by Romney's surging support in suburban Philadelphia, announced Tuesday that the campaign would also begin buying its own ads in Pennsylvania.

"Mitt Romney is more competitive in the voter-rich Philadelphia suburbs than any Republican nominee since 1988," Beeson said. "This makes Pennsylvania a natural next step as we expand the playing field."

The move by Romney and the PACs supporting him forced Obama's campaign to dump $600,000 into Pennsylvania media advertising, though Democrats insist they're still confident of an Obama victory in the Keystone State.

Vice President Biden was set to visit his boyhood hometown of Scranton, Pa., on Thursday but had to scrap the trip because of Hurricane Sandy.

In a memo to supporters, Obama's Pennsylvania campaign director, Bill Hyers, noted that Democrats enjoy a nearly 2-to-1 advantage in new voter registration in the state. And among existing voters, Democrats outnumber Republicans 4.25 million to 3.13 million, he said.

"And this margin has been growing in the months ahead of the registration deadline," Hyers added.