ABC News’ Rick Klein reports: Rep. Joe Sestak, D-Pa., has decided to take on Sen. Arlen Specter, D-Pa., in next year’s Democratic primary — despite efforts by the White House and Pennsylvania’s political establishment to clear the field for the newest Democratic senator. A Sestak aide confirms to ABC that the congressman has decided to jump into the race. Sestak, who represents the Philadelphia suburbs, today started a three-week tour of Pennsylvania’s 67 counties with an interview where he declared his intent to run against Specter. “I am going to get into the race against Arlen Specter in the Democratic race for senator,” Sestak told the Wayne Independent. Sestak’s candidacy, while long expected, sets up a showdown in a critical state that pits the White House and most of Pennsylvania’s major Democratic elected officials against liberal elements of the party, who are rallying support against Specter — who was a Republican until two months ago. Also a factor: Organized labor, which is threatening to withhold support from Specter if he doesn’t support labor’s top legislative priority — a measure, known as “card-check,” that would make it far easier for unions to form. The Obama White House has offered its “full support” to Specter, though it’s unclear what that will mean in a competitive primary. One Pennsylvania Democratic operative tells ABC that plans are already underway to have the president headline a fundraiser this fall for Specter, though no plans have been formally announced. The winner of the Democratic primary is likely to face former Rep. Pat Toomey, R-Pa., who fell just short of defeating Specter in the Republican primary in 2004. Republicans have tried and failed to recruit other candidates — most notably former Gov. Tom Ridge — for Specter’s seat.<p>Congressman Sestak</p> UPDATE: A Sestak spokesman issued a statement this afternoon amending slightly what his boss said this morning – to maintain that no final announcement has been made. “He does, as he has said recently, intend to get in the Senate race as he finalizes his decision with his family, who are very supportive,” said spokesman Jonathon Dworkin.