Arlen Specter, who switched parties last year to avoid a primary duel with a more conservative candidate, faces a tough election.
Both candidates will crisscross Pennsylvania on Monday as they make their last pitch to voters before Tuesday's Democratic primary.
Specter is flying to events in Erie, Johnstown, Harrisburg, Scranton, the Lehigh Valley and finally Philadelphia. Sestak is greeting voters in Pittsburgh and attending rallies later in his southeastern Pennsylvania home base.
A poll released Monday shows the race is too close to call, the latest of many tough election challenges for the fifth-term Specter.
Specter has the Democratic establishment's backing after switching parties last year to avoid a GOP primary battle with a more conservative candidate. Sestak says Specter is a political opportunist interested only in saving his job.