Longtime Sen. Arlen Specter

Republican presidential candidate Ronald Reagan holds up the hand of U.S. Senate candidate Arlen Specter at fundraising party for Specter in Philadelphia.  (Walter Zeboski / Associated Press)

Dr. Everett Koop, left, who had been nominated to be the U.S. surgeon general, chats with Sens. Arlen Specter, center, and Orrin Hatch (R-Utah) before a confirmation hearings on Capitol Hill.  (Ron Edmonds / Associated Press)

Senate Judiciary Committee member Arlen Specter questions witnesses defending Anita Hill, who accused Clarence Thomas of sexually harassing her in the 1980s.  (John Duricka / Associated Press)

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

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)

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)

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)

President Obama greets Sen. Arlen Specter at an event honoring Jewish American Heritage Month in the East Room of the White House.  (Charles Dharapak / Associated Press)