In a recent sketch on Saturday Night Live, the NBC comedy show, an actor playing Joe Biden gave a grim description of life in Scranton, Pennsylvania, where Barack Obama's running mate grew up.

"It's just an awful, awful sad place, filled with sad, desperate people with no ambition," he said. "So don't be telling me that I'm part of the Washington elite, because I come from the absolute worst place on earth."

In reality, Mr Biden has worn his Scranton roots as a badge of honour on the campaign trail, as the hard-pressed former coalmining town has emerged as Ground Zero in the battle for blue-collar voters.

Mr Biden and his Republican counterpart, Sarah Palin, each drew thousands of supporters to separate rallies in Scranton this week, highlighting its starring role in the election as symbol of working-class America.

"To use a baseball analogy, Scranton is a place where people are focused on hitting singles just to pay the bills," says Christopher Doherty, the town's Democratic mayor. "Nobody's even thinking about hitting a home run."

Scranton rose to prominence during the Democratic primaries when Hillary Clinton invoked her family ties to the town - her grand­father worked in one of its former lace mills - as evidence of her blue-collar ­values.

Until recently, it looked as if Mr Obama's weakness in places such as Scranton could cost him the election as he struggled to win over the white working-class Democrats who backed Mrs Clinton for the nomination.

Over the past month, however, blue-collar America appears to have swung strongly towards the Illinois senator as economic concerns trump the cultural and national security issues that John McCain hoped would carry him to victory.

Nowhere has the shift been clearer than in Pennsylvania, where increased support from lower-income whites has helped Mr Obama open a double-digit advantage in one of Mr McCain's top target states.

In early September, a Quinnipiac poll gave Mr McCain a 7 percentage point lead among non-college-educated whites in Pennsylvania. A month later, he trailed by 3 percentage points among the same group.

Mr Obama still trails among white, working-class men but the gap has narrowed, while the Democrat has opened a strong lead among lower-income white women - the so-called "Wal-Mart Moms" identified by pollsters as this year's crucial swing group.

Mrs Clinton, who beat Mr Obama by 9 points in the Pennsylvania primary, has rallied to her former rival's cause by campaigning across the state on his behalf this week - including a joint "homecoming" event with Mr Biden in Scranton. "This election is too important to sit on the sidelines of history," the former first lady told 6,000 supporters in a local sports hall.

Stephen Ruddy, a 51-year-old demolition worker and former Clinton supporter from Scranton, plans to vote Democrat despite misgivings about the nominee. "It will stick in the throat to support Obama but I'm a working man and we can't afford another four years of Republicans running the show," he said, adding that his union was putting members under pressure to fall into line.

Ed Rendell, governor of Pennsylvania and a former Clinton ally, says the financial crisis has neutralised fears among some white voters about electing America's first black president. "If you're drowning, and you're in the middle of the river and you see a guy on the riverbank, and he's got a coil of rope, you don't care whether he's black, white, green, purple," he told Fox News. "All you care about is whether he has a strong enough arm to get that rope out to you in the middle of the river. Barack Obama's got that strong arm."

If energy and healthcare are included, three-quarters of Pennsylvania voters named economic issues as their top election concern in the most recent Quinnipiac poll, pushing conservative causes such as abortionand gun control to the margins.

That explains why Ms Palin spent most of her speech to 5,000 supporters in Scranton on Tuesday delivering a populist economic message, drawing back from recent personal attacks against Mr Obama that polls suggest have backfired.

Many Republicans voiced scepticism about Mr Obama's poll lead and confidence that Mr McCain could still win Pennsylvania. "There haven't been jobs here for 20 years, so the economy doesn't change anything," said Anne Marie Knott, a 42-year-old mother of five. "People in Scranton still put family values first."

But Terry Madonna, political scientist at Franklin & Marshall College in Lancaster, Pennsylvania, says the McCain campaign needs more than the faith of its supporters to justify its heavy investment in the state. "They are spending a lot of time and money in a state that looks to be slipping away," he says.