After three months of unofficial campaigning against Arlen Specter, U.S. Rep. Joe Sestak will make a major campaign announcement Tuesday. It is expected that Sestak will make official his bid to upset Specter in a 2010 Democratic primary for U.S. Senate.

Sestak will make his announcement at 8:30 a.m. at the Herbert W. Best VFW Post in Folsom, Pa. The event will be streamed online at www.joesestak.com and followed by press conferences in Pittsburgh and Johnstown.

On Wednesday, Sestak will be in Harrisburg and Scranton, before finishing as a guest on 'The Colbert Report.'

Sestak, 57, is a former U.S. Navy admiral in his second term in Congress. Sestak opposes Democratic Party officials who 'anointed' Specter for re-election and has made a case for expanding the choice for Democrats in the 2010 primary.

Sestak's unofficial candidacy has apparently had an impact on Specter, the 79-year-old former Republican. Since switching parties on April 28 and facing the potential showdown against Sestak, Specter has voted almost entirely along Democratic Party lines.