Arlen Specter

Sen. Arlen Specter, left, confers with Senate Judiciary Committee Chairman Patrick Leahy (D-Vt.) in 2002 at a hearing about government surveillance powers.  (Joe Marquette / Associated Press)

Specter and his wife, Joan, greet supporters after 2004 primary elections. Specter defeated challenger Pat Toomey, a conservative. Toomey is expected to challenge Specter again next November, when the former Republican runs for reelection as a Democrat.  (George Widman / Associated Press)

As Senate Judiciary Committee Chairman, Specter, center, listens to testimony on detainee and enemy combatant legal rights in July 2006. He’s flanked by Sens. Orrin Hatch (R-Utah) and Patrick Leahy.  (Dennis Cook / Associated Press)

Specter talks to reporters before voting on the economic stimulus bill in February 2009. He was one of three Republican senators to cross party lines and support the $787-billion package -- the three votes were just enough to ensure that the GOP could not block the bill with a filibuster.  (Chip Somodevilla / Getty Images)

Vice President Joe Biden talks with Specter.  (Saul Loeb / AFP/Getty Images)

Pennsylvania lawmakers surround Specter at a University of Pennsylvania reception to honor his vote supporting the economic stimulus package.  (Matt Rourke / Associated Press)

Then-Sen. Arlen Specter in his office in 2009.  (Nancy Stone / Chicago Tribune)

Specter speaks at a Senate Appropriations Committee emergency hearing about swine flu on Tuesday, the day he announced he was leaving the Republican Party.  (Tim Sloan / AFP/Getty Images)

Sen. Tom Harkin (D-Iowa) greets Specter. Specter ran for reelection to the Senate as a Democrat in 2010. “I have found myself increasingly at odds with the Republican philosophy and more in line with the philosophy of the Democratic Party,” he said.  (Michael Reynolds / EPA)