U.S. Sen. Arlen Specter walks with his wife, Joan, to an emergency hearing on swine flu by the Agriculture, Nutrition and Forestry subcommittee Tuesday in Washington. Specter, 79, switched to the Democratic Party on Tuesday.

WASHINGTON - For decades, Arlen Specter was the kind of Republican who would rather fight than switch.

A moderate in a party that has grown increasingly conservative, the Pennsylvania senator pushed, provoked and preached for a broader, more inclusive GOP.

On Tuesday, he gave up that battle, citing irreconcilable differences. In a bid to save his own Senate career, Specter cast his lot with the Democratic Party, where he'd begun his political life more than four decades ago.

The move was vintage Specter: the 79-year-old cancer survivor and political survivor declaring himself "ready, willing and anxious to take on all comers."

Over a 28-year Senate career, Specter has managed to rankle all sides with his independence and sometimes prickly personality. Pennsylvania's longest-serving senator is a former prosecutor who earned the nickname "Snarlin' Arlen."

He irked women with his prosecutorial treatment of Anita Hill during Supreme Court confirmation hearings in 1991 when she accused Clarence Thomas of sexual harassment. Four years earlier, he irked Republicans by siding with Democrats to sink the nomination of Judge Robert Bork to the high court.

And he wore his battle scars like a badge of honor.

"One of my colleagues once said I had the keen capability to alienate the entire electorate in just two votes," Specter said in a speech years later, and he tossed out a similar line on Tuesday at a news conference announcing his party switch.

Specter, who made his own ill-fated presidential bid in 1996, stuck his neck out again earlier this year when he was one of just three Republicans in the Senate to support President Barack Obama's $787 billion stimulus package.

"When these tough votes come up, it's a lot more comfortable to keep your head below the trench line and be inconspicuous and let somebody else do the tough work, and there wasn't anybody else to do it," he later explained.

In the end, Specter said, the stimulus vote opened a schism between him and the GOP that couldn't be closed.

A Democrat until his first political race for Philadelphia District Attorney, Specter switched to the GOP to challenge his former boss in 1965.

He was upfront Tuesday about the political calculations behind his latest maneuver, calling his chances of surviving a Republican primary next year "bleak."

GOP consultant Roger Stone, a longtime friend and unpaid adviser to the senator, said the switch wasn't an easy decision for Specter, adding that the senator "liked being a Republican."

"He's always wanted to stay and fight," Stone said. "I'm sure he anguished about this, but it's survival."

Specter, a Kansas native, started out as a lawyer in private practice before becoming an assistant prosecutor in Philadelphia. In 1964, he became a staff lawyer for the Warren Commission, where he pushed the theory that a single bullet hit both President John Kennedy and Texas Gov. John Connally. His work led the commission to conclude that Lee Harvey Oswald was the lone assassin.

Throughout his career, Specter has demonstrated not just political stamina but physical resilience. He's gone multiple rounds with cancer, and last year wrote a book titled, "Never Give In: Battling Cancer in the Senate," in which he credited hard work with helping him get through chemotherapy.

Pennsylvania Gov. Ed Rendell, a Democrat who once worked under the senator, once said Specter came through the cancer battle as "the same irrational, difficult, ornery person that he always was." And that was meant as a compliment.