[cnn-photo-caption image= http://i2.cdn.turner.com/cnn/2009/images/04/30/art.sestak.gi.jpg caption="Rep. Joe Sestak officially declared his candidacy for the the Democratic Senate nomination in Pennsylvania."]
WASHINGTON (CNN) – Rep. Joe Sestak officially declared his candidacy for the the Democratic Senate nomination in Pennsylvania Tuesday, setting up a primary season showdown with incumbent Sen. Arlen Specter.
Although his campaign team has been sharply critical of Specter since he began considering a run, Sestak focused on policy matters in his announcement - the economy, health care and other challenges.
"These problems, when I am your senator, are not going to just disappear overnight - I can't promise you that," he said. "But I can promise you that you will have working for you the hardest working senator. That you will have the most honest of senators, the most accountable of senators. You will have the most caring of senators, and you will have the senator with the most energy."
As for Specter: "We are where we are today, and if everybody's happy with that, he's your guy."
Much of Sestak's chance of success is based on the idea that he represents a pure progressive alternative to Specter - a candidate who could peel away disaffected liberal voters unhappy with the prospect of casting their ballot for Specter, the former Republican.
Sestak makes a pitch to these voters in a video on his campaign's Web site - the lawmakers Pennsylvania have sent to Washington, he says, have failed the state's residents.
Shortly after the announcement, which took place at a VFW hall in his district, the Philadelphia-area congressman sent a fundraising message to supporters, a move that underscored one of the massive hurdles he'll have to clear to capture his party's nod.
Sestak trails Specter in the money chase, but he boasts a sizeable campaign war chest: At the end of June, Sestak had just over $4 million in the bank, around $3 million less than the incumbent.
And a Quinnipiac poll released late last month indicated that Specter, who's received the public backing of President Obama and party leaders since his defection from the Republican Party this spring, still holds a greater than 2-to-1 advantage over Sestak, with a 55 percent to 23 percent advantage in a hypothetical Democratic primary matchup.
The same poll also suggested that Specter's 20-point edge over prospective GOP challenger Pat Toomey had vanished.