Sen. Hillary Clinton will join vice presidential candidates Joe Biden and Sara Palin at rallies in a state that has been a Democratic stronghold for 20 years, but is considered a potential upset for McCain.

"We pay attention to our own polls," Mark Salter, a senior McCain aide, told reporters this weekend. "Pennsylvania looked much better than Michigan."

Politico pointed out Sunday that Pennsylvania was solidly in Clinton's camp during the primary, and Palin has been hard at work courting the state's working-class communities with the message that Obama is an elitist liberal who associates with shady characters.

Obama himself was on Philadelphia Saturday to whip up enthusiasm, and his campaign has outspent McCain on television advertising in recent days in order to reduce the chances of a GOP upset in a state with 21 electoral votes.