Specter is seeking a sixth term in the U.S. Senate and heading into his first election as a Democrat. He left the Republican party three months ago.
Sestak, a former Navy admiral, kicked off his campaign at his VFW post in Folsom in the Philadelphia suburbs, The Philadelphia Inquirer reported. He called for "change and accountability."
Specter's campaign manager, Christopher Nicholas, attacked Sestak's record in Congress.
"He hardly deserves a promotion since he has missed 105 House votes this year, which is the worst attendance record of any Pennsylvania congressman," Nicholas said.
Recent polls have shown Specter with a comfortable lead over Sestak in the primary and in a tight race with former Rep. Pat Toomey, the most likely Republican challenger. Toomey almost unseated Specter in the 2004 Republican primary.
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