Sestak, 57, a retired admiral who unseated 10-term Republican U.S. Rep. Curt Weldon in 2006, has strongly criticized how President Obama and other Democrats swiftly "anointed" Specter as their nominee when he switched parties in April.
Earlier this year, when Specter was still a Republican, Sestak said that he'd been asked by the Democratic Senatorial Campaign Committee to consider challenging Specter.
Now, nobody can talk him out of it, to the dismay of party leaders from Philadelphia to Washington, D.C.
The second-term congressman began his day at the VFW, where "Democrat for Senate" lawn signs had been stapled on top of his old congressional campaign signs, then headed to Pittsburgh and Johnstown. He plans to hit Harrisburg and Scranton today before appearing on Comedy Central's "The Colbert Report," which will air at 11:30 tonight.
If elected, Sestak said, he would support Obama's agenda and help build an economy that benefits the working class, not just the "well-to-do." His campaign platform will include improving accessibility to affordable health care and education.
"We're Americans," he said. "We can do that."
A notorious workaholic, Sestak broadsided the Washington political establishment for letting the economy run aground.
"Some people were asleep at the switch," Sestak said. "Those people who you sent to Washington to represent you, to look out for you, failed to do so, and they must be held accountable."
"Give 'em hell, Joe!" a supporter yelled.
Speaking with reporters afterward, Sestak brushed aside the wishes of Democratic leaders - including Gov. Rendell - who have sought to clear the field for Specter.
"Boy, if Barack Obama had listened to that, we wouldn't have a first-term senator being president today," he said.
