Still, many Democratic primary voters had never voted for Specter, and they weren't about to start. Instead, they picked his primary opponent, then-U.S. Rep. Joe Sestak, despite Specter's endorsement from Obama, Rendell and Biden.
Born in Wichita, Kan., on Feb. 12, 1930, Specter spent summers toiling in his father's junkyard in Russell, Kan., where he knew another future senator — Bob Dole. The junkyard thrived during World War II, allowing Specter's father to send his four children to college.
Specter left Kansas for college, graduating from the University of Pennsylvania in 1951 and Yale law school in 1956. He served in the Air Force from 1951 to 1953. After working on the Warren Commission, he returned to Philadelphia and wanted to run for district attorney in 1965. But he found that he would have to challenge not only his boss, but the city's entrenched Democratic Party. Specter ran as a Republican and won.
Friends say his childhood circumstances made him determined, tough and independent-minded. In his 2000 book, "Passion for Truth," he noted how his father had complained bitterly that the U.S. government had broken its promise to pay a bonus to World War I veterans.
"Figuratively, "he wrote, "I have been on my way to Washington ever since to get my father's bonus."
For Specter, the benefit of crossing party lines wasn't always about being true to his convictions. He also used it to benefit the causes he championed.
"He was a master politician," Rendell said. "He was as smart as a whip."
In 2001, he voted for President George W. Bush's package of tax cuts, but voted with Democrats to route $450 billion into education and debt reduction. He negotiated $10 billion for medical research when he agreed to vote for the stimulus.
But Specter also believed in the political middle, and often lamented the disappearance of moderates who had the courage to buck party leadership.
In one study of congressional polarization, University of Georgia professor of political science Keith Poole mapped the political polarization of Congress by charting votes and found that the parties are more divided than at any time since Reconstruction after drifting further apart in the last 40 years.
Poole said in an essay that there are no true moderates left in the House of Representatives, and just a handful remaining in the Senate, in contrast to the Reagan era when about half of the members of Congress could be described as moderates.
The other two Republicans who supported Obama's stimulus are Maine's two U.S. senators. One of them, Olympia Snowe, announced in February that she wasn't seeking re-election. She said she was frustrated by "'my way or the highway' ideologies."
Specter's funeral was scheduled for Tuesday in Penn Valley, Pa., and will be open to the public, followed by burial in Huntingdon Valley, Pa.
Besides his son, Shanin, Specter is survived by his wife, Joan, son, Steve, and four granddaughters.