[cnn-photo-caption image= http://i2.cdn.turner.com/cnn/2009/images/04/30/art.sestak.gi.jpg caption="Rep. Joe Sestak is expected to officially declare his candidacy for the Democratic Senate nomination."]WASHINGTON (CNN) - Rep. Joe Sestak is expected to officially declare his candidacy for the Democratic Senate nomination Tuesday, setting up a primary season showdown with incumbent Sen. Arlen Specter, a source close to Sestak confirmed to CNN Monday.
Sestak's campaign said in a Twitter message Monday morning that the Philadelphia-area congressman will be making a "major announcement" Tuesday at 8:30 a.m. The announcement, which will take place at a VFW hall in his district, will be webcast live on his campaign site.
A Quinnipiac poll 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, had a 55 percent to 23 percent advantage in a hypothetical Democratic primary matchup – though his 20-point edge over prospective GOP challenger Pat Toomey had vanished.