Philadelphia's four suburban counties, home to one in five Pennsylvania voters, are the focus of much attention from U.S. Rep. Joe Sestak, a Democrat, and former Congressman Pat Toomey, a Republican, in the final days of the campaign.
Registration among the 1.7 million voters in the four "collar" counties - Bucks, Chester, Delaware and Montgomery - is nearly balanced between Democrats and Republicans.
Sestak, wearing a green jacket with an emblem from the aircraft battle group he commanded in his Navy career, hit populist notes to fire up a few dozen canvassers in a Doylestown parking lot, targeting what he characterizes as Toomey's corporate favoritism.
"Just make sure everyone comes out for us, for the people - we the people, not the corporations," he said.
Sestak was introduced by U.S. Rep. Patrick Murphy - an Iraq war vet who himself is in a tight contest to win a third term - and many of the lawn signs peppering the area play up their military service: "We're voting for the paratrooper and the admiral."
In another part of Bucks County, Toomey spoke to more than 200 people in a Bristol banquet hall, before heading to a Delaware County ice rink on the other side of Philadelphia in Sestak's congressional district.
"The southeast is a swing area, where voters - you know how this goes, it's every election cycle - voters in some cycles they'll vote for Democrats, other times they'll vote for Republicans," Toomey told a seated crowd of about 150. "This Tuesday, I need to carry Delaware County. If I carry Delaware County, I'll win this race."
Toomey has ridden a wave of voter dissatisfaction over joblessness and the policies of President Barack Obama, including the new federal health care law.
Along with turnout in heavily Democratic Philadelphia, the campaigns will closely watch the tally in the suburbs, where Democratic registration has increased in recent years.
Toomey and Sestak are competing to succeed five-term Sen. Arlen Specter, whom Sestak beat in the May primary.
Beginning in 2000, no candidate in a general election race for president, U.S. senator or governor has lost the four suburban counties, but won the election.
Pollsters characterize the counties' voters as fiscally conservative, but liberal to moderate on abortion rights and environmental causes.
With the economy weighing heavily on voters, Toomey and Sestak have portrayed each other as supporting fiscally reckless policies while each was in Congress.
They disagree on abortion rights - Sestak favors them, Toomey opposes them and would support overturning the Roe v. Wade Supreme Court decision.
On environmental issues, Sestak supported a measure in the U.S. House to discourage using fuels that generate the greenhouse gases scientists have linked to global warming.
Sestak said the so-called "cap-and-trade" measure would invigorate the growth of a "green" economy and jobs. But Toomey said it would raise energy costs and deliver a fatal blow to Pennsylvania's steel and coal industries.
Toomey said he supports government investment in the development of cleaner fuels, but opposes any effort that he said would punish the use of dirtier fuels.
The race is close, with some polls giving Toomey a slight lead, and others showing the candidates running even. Sestak, who was elected to Congress in 2006, and Toomey, who represented the Allentown area in Congress from 1999 to 2005.