While the Democrats remain in a tight race for their presidential nomination, Sen. John McCain (R-Ariz.) has turned his attention to the general election.

Sen. Hillary Clinton (D-N.Y.), of Chappaqua, and Sen. Barack Obama (D-Ill.) put in another day of campaigning in Pennsylvania Monday. As the April 22 primary approaches, Clinton pledged to stay in the race.

"There are some folks saying, well we oughta stop these elections," Clinton said. "I didn't think we believed that in America," she said referring to Obama's lead in pledged delegates.

Obama, who also leads Clinton in the number of contests won, welcomed Clinton's opposition. "My attitude is that Senator Clinton can run as long as she wants," Obama said. "Her name is on the ballot, and she's a fierce and formidable competitor. And she obviously believes that she would make the best nominee."

Some political insiders have said it's mathematically impossible for Clinton to gain the 2,024 delegates needed to win the Democratic presidential nomination before the party's August convention. However, others have said it's just too close to call, saying superdelegates may make the decision.

Meanwhile, McCain has rolled out his "Service to America" tour, despite not knowing who he'll face in the general election. He's hitting the campaign trail on a five-state bus tour to reintroduce himself to voters.

McCain's camp has also released the senator's first general election TV ad.