Until his dramatic announcement that he was switching parties, Sen. Arlen Specter (Pa.) spent nearly 45 years polishing his Republican credentials.

But Tuesday’s announcement wasn’t the five-term Senator’s first time switching parties.

The 79-year-old Specter was a Democrat until he was 35 years old, and a well-known one at that, at least in Philadelphia.

A Yale Law School graduate, Specter had been an assistant Philadelphia district attorney who led the prosecution of Teamsters Local 107, a corruption case that garnered the attention of then-Attorney General Robert F. Kennedy, who chose him for a slot on the Warren Commission.

There, he played a central role in developing the single-bullet theory.

When Specter returned to Philadelphia, then-Gov. William Scranton (R) appointed him to lead a special task force investigating the city’s corrupt judicial magistrate system.

It was the perfect launching pad for a run at district attorney, but, Specter recalled in a 2004 interview, the chairman of the Democratic City Committee, Frank Smith, pointedly told him: “We don’t want a young Tom Dewey.—

“So, then the Republicans offered me the nomination, no strings attached. The nomination wasn’t worth very much, but I shook every hand in town and won.—

Two years later, in 1967, he ran for mayor of Philadelphia, losing by just 11,000 votes.

In 1969, he won a second term as Philadelphia DA, the last election he would win in 11 years.

He lost his third term for DA in 1973, and he was defeated in the 1976 Republican Senate primary by John Heinz and in the 1978 GOP gubernatorial primary by Richard Thornburgh.

He finally won a Senate seat in 1980 — with just 50 percent of the vote.

Specter has always been something of an outlier, a Jewish Republican who often bucked his party — on the Comprehensive Nuclear Test Ban Treaty and minimum wage bill in 1999, on the federal tobacco lawsuit in 2000, and overtime regulations in 2003 and 2005, according to the Almanac of American Politics.

Of course, the longer Specter stayed in office, the more “liberal— he became, at least in relationship to the rest of his party.

When he was first elected, filling the seat of retiring GOP Sen. Richard Schweiker, he became part of a group of at least a dozen other like-minded moderate-to-liberal Republicans, from Bob Packwood and Mark Hatfield of Oregon to Alan Simpson (Wyo.), Lowell Weicker (Conn.), Charles Percy (Ill.), Charles Mathias (Md.), John Danforth (Mo.), Rhode Island’s John Chafee and his Pennsylvania colleague John Heinz.

By 2009, he was virtually alone, with Maine’s two Republicans, Sens. Olympia Snowe and Susan Collins, as occasional company.

Indeed, Specter grew up as an outsider. His father was a Jewish immigrant from the Ukraine with no formal education; his mother came from a small town near the Russian-Polish border.

In 1942, when Specter was 12, the family moved to Russell, Kan., the hometown of former Senate Majority Leader Bob Dole (R). Specter’s father had a scrap metal business recycling iron from oil derricks. He weighed his metal at the Russell Grainery, which was run by Dole’s father.

They were the only Jewish family in the town.

In nearly 30 years in the Senate, Specter has had several eye-catching moments.

In 1987, he helped derail the nomination of Robert Bork for U.S. Supreme Court, a move that earned him the eternal scorn of the conservative right.

Four years later, in a move that angered the liberal community just as much, he supported Supreme Court nominee Clarence Thomas, publicly questioning the veracity of Anita Hill in televised hearings.

Washington Post television critic Tom Shales called Specter’s interrogation of Hill “merciless, snide, supercilious.— Specter duly quoted Shales’ comments in his memoir, “Passion for Truth.—

In 1995, Specter was again smack in the public eye, when his Judiciary subcommittee held hearings on the 12-day standoff at Ruby Ridge, Idaho, between the family of Randy Weaver and the FBI.

And during the impeachment trial of President Bill Clinton, Specter invoked Scottish law, choosing to vote “not proven and therefore not guilty.—

In February 2005, Specter was diagnosed with advanced Hodgkin’s disease, although he continued to chair the Senate Judiciary Committee throughout his treatments. In fact in many respects, his ascension that year to the chairmanship of Judiciary, a panel on which he has served his entire time in the Senate, has been his crowning achievement.

And even that accomplishment was mired in self-inflicted drama. At a press conference on Nov. 3, 2004, after a grueling primary and a tough general election, Specter said it was “unlikely— that the Judiciary Committee would confirm any judicial nominee who would overturn Roe v. Wade.

The uproar among conservative groups was immediate, and Specter was forced to furiously backpedal, publicly insisting that he would not use a litmus test and was only stating the obvious.

Two weeks later, surrounded by now-smiling Republican colleagues, Specter issued a formal statement reiterating his GOP bona fides.