Vice presidential candidates Sen. Joe Biden and Gov. Sarah Palin campaigned in central and western Pennsylvania Thursday and Friday, as the campaigns continued to battle for Pennsylvania’s 21 electoral votes in the final days of the election.

Palin spent Thursday and Friday on a bus tour across the state, beginning in Erie in northwestern Pennsylvania and ending in York, in the central part of the state. She nearly intersected with Biden Thursday in Williamsport, where the two held rallies just three hours apart.

Both campaigns consider the Keystone state key to their electoral hopes. Pennsylvania has not voted for a Republican presidential candidate since 1988, but President Bush lost to Democratic Sen. John Kerry in 2004 by only 2.5 percentage points.

And with Obama looking strong in several 2004 red states, including Virginia and Ohio, the McCain campaign must look to shift Pennsylvania from blue to red to offset those potential losses.

“McCain really has no choice but to give Pennsylvania a lot of attention, because it is the only fairly large state that Kerry won where McCain has any real chance of winning,” University of Pennsylvania political science professor Rogers M. Smith told CBS News. “And there is no path to 270 electoral votes in which he fails to take any Kerry states, because Obama is leading in some of the Bush states pretty decisively.”

On Friday the Web site RealClearPolitics put Obama ahead 9.3 percent in Pennsylvania in its average of the latest polls. But individual polls varied, with a CNN/Time poll putting Obama up 12 percent and an NBC/Mason-Dixon poll putting him only 4 points ahead of McCain.

Obama lost the Pennsylvania primary to Sen. Hillary Clinton by 10 points, mostly because Clinton won working-class white voters in the western part of the state. But Obama and Biden have been campaigning hard across the state, and are expected to do well in the Philadelphia area.

“Forty percent of the state’s voters live within the reach of the Philadelphia television market,” Franklin and Marshall College political science professor G. Terry Madonna told CBS News. “If you win 60 percent where 40 percent live, you don’t need to do much elsewhere.”

Meanwhile, Pennsylvania’s election officials are preparing for a busy Election Day. On Thursday, a federal judge ruled that officials must offer paper ballots to voters any time more than 50 percent of voting machines break down at a polling place.

The state NAACP, together with the Election Reform Network and three individual voters, brought a suit to overturn a directive by Pennsylvania’s Secretary of the Commonwealth, Pedro A. Cortes, that would have allowed the use of paper ballots only when all machines at a polling place were broken.

The judge, Harvey Bartle III, agreed with the plaintiffs, writing: “Based on the record before us, we find that there is a real danger that a significant number of machines will malfunction throughout the Commonwealth, and this occurrence is likely to cause unacceptably long lines on Nov. 4 […] We sincerely hope this scenario will not occur, but we cannot allow our decision to be based on hope.”