WASHINGTON -- In nearly 30 years as a Senate Republican, Arlen Specter was tough to pin down, frequently breaking ranks with his party and giving its leadership fits.

Yet in the half-year since he crossed the aisle, Mr. Specter has become a remarkably reliable Democratic vote.

Wednesday marks the six-month anniversary of Mr. Specter's party switch, which came after he sided with Democrats to approve the controversial stimulus bill and he knew that he wouldn't win a primary race for a sixth term as a Republican.

Since then, Mr. Specter has been lauded by Democrats and has seen his campaign coffers swell with help from President Barack Obama and Vice President Joe Biden -- who have campaigned with him.

But his prized seniority on committees has been stripped, as has his status as a coveted swing vote. As leadership and the media lavish attention on moderate Republicans Olympia Snowe and Susan Collins of Maine, and centrist Democrats such as Ben Nelson of Nebraska, Mr. Specter speaks out in favor of the "public option" and is considered a sure vote when a merged health care bill hits the Senate floor.

According to a Congressional votes database compiled by The Washington Post, Mr. Specter has voted with the Democrats 94.2 percent of the time since switching parties -- an identical figure to his fellow Pennsylvania Democrat Bob Casey Jr. and in the middle range of his party's caucus.

Since Rep. Joe Sestak, D-Delaware County, joined the race in late May, Mr. Specter has voted even more consistently with his party: a 96.4 percent rate, excluding a handful of votes on which he abstained.

"There has not been a vote that I felt constrained to object to," Mr. Specter said last week.

When he was a Republican, Mr. Specter was a frequent objector. A tally of divisive votes by Congressional Quarterly showed that, as of mid-August, Mr. Specter had a "party unity" score of 90.5 percent with the Democrats -- far higher than he ever had with the Republicans. The Washington-based publication calculated that Mr. Specter averaged a 58 percent party unity score from 1981 through 2008.

Now in the midst of a primary fight, Mr. Specter trumpets those votes to argue that he was really a Democrat all along.

"I've been over there a lot in a sense all during my career because I've really voted with the Democrats more often than with the Republicans," Mr. Specter said, recalling his support for stem cell research, children's health insurance and several other instances of breaking ranks with Republicans.

"He still has his independent streak -- as we all do," said Sen. Pat Leahy, D-Vt., chairman of the Judiciary Committee. "He's spoken up in caucuses, and people have been listening to him."

Mr. Leahy added that the Democratic leadership has been quite pleased with Mr. Specter so far.

Mr. Sestak's campaign has launched a Web site intended to remind Democratic primary voters that Mr. Specter was, in fact, a Republican. It touts votes for the Iraq war and President George W. Bush's tax cuts. "My take is, it's of interest that he's following my leadership in voting the way he is," Mr. Sestak said.

"But the issue is: What are you for? And will you be there tomorrow? ... [In] 30 years in Washington, D.C., he learned in the Republican leadership how to make deals, how to survive."

The son of immigrants, Mr. Specter was raised in a household of Franklin Delano Roosevelt Democrats. But his experience in the Philadelphia district attorney's office in the early 1960s turned him off to the city's party machine. When he decided to run for district attorney in 1965, his path as a Democrat was blocked so he ran as a Republican.

The decision was not easy, he wrote in his 2000 autobiography, "Passion for Truth." He stuck with the party for 44 years, but the bonds frayed in February, when Mr. Specter joined Ms. Snowe and Ms. Collins as the only three Republicans to back Mr. Obama's controversial stimulus bill.

Mr. Specter called the vote the most important he's ever cast, saying it was vital to keep the nation from sliding into a Depression.

Republicans disagreed.

"They filed a resolution of censure against me in the state party," Mr. Specter said. "They picketed my offices. There were irreconcilable differences, there's no question."

A March 25 Quinnipiac poll showed Mr. Specter trailing former Rep. Pat Toomey, of Allentown, by 14 points in the Republican primary. Mr. Specter narrowly defeated Mr. Toomey in the primary in 2004.

So on April 28, Mr. Specter changed parties, though he said he did not feel the same misgivings that accompanied his last party switch. He said his concerns then mostly related to his father, a staunch Democrat.

Mr. Toomey said he was surprised, but in retrospect, he shouldn't have been.

"I think he'd join any party and take any position if it would improve his chances of holding onto the seat," Mr. Toomey said.

The move gave Senate Democrats a crucial 60th vote needed to end debate on bills, but Mr. Specter quickly pledged not to be an automatic vote with leadership.

Appearing May 3 on NBC's "Meet the Press," Mr. Specter said he would be opposed to a government-run public option in health care reform. But in TV appearances and an op-ed on the Huffington Post Web site, Mr. Specter now is a strong proponent of the public option.

Mr. Specter said last week that in his appearance on NBC, he was thinking of a plan like the one pitched by former President Bill Clinton, which Mr. Specter had a major hand in defeating.

"I really had in mind the Clinton health care plan, which had an enormous bureaucracy between doctor and patient -- and I'm not for that," Mr. Specter said.

"But to have it as an option, this is not single-payer, this is a plan which is on a level playing field. So it's competing against private companies. The government's not taking over health care. It's been badly misinterpreted, just not accurately portrayed. So there's really been no change in my position."

Mr. Specter also has been attacked for shifting position on the Employee Free Choice Act, a bill that could expand the labor force and has massive union backing. He said he was against it early in 2009, while he was still a Republican.

Since the party switch, Mr. Specter has said he supports the bill but not its so-called "card check" provision, which would allow union organizing without a secret ballot. In September he told the AFL-CIO convention in Pittsburgh a compromise had been reached in the Senate. But there has been no action on the bill; most controversial pieces of legislation have been placed on the back burner during the health care debate.

While the party switch touched off external tumult, Mr. Specter said he's seen no personal fallout in the exclusive club that is the Senate. He remains cordial with Republican colleagues, including leadership.

"The Senate understands a party change," he said.

Because he does maintain relationships with Republicans, Mr. Specter has touted himself as a consensus-builder. Last week, he introduced a bill -- co-sponsored by Sen. Lindsey Graham, R-S.C. -- to increase penalties for Medicare and Medicaid fraud. And he said he is working to bring in the other two members of the stimulus trio on health care.

The party change has deprived Mr. Specter, former chair of the Judiciary, Veterans Affairs and Intelligence committees, of seniority within the caucus. Senate Democrats assigned him to spots among the more junior members of his committees: judiciary, veterans' affairs, environment and public works, appropriations and the special committee on aging.

"I think the seniority is going to be revisited, I think fairly soon," Mr. Specter said, adding that he means before the 2010 elections.

Though polls show he maintains a healthy lead on Mr. Sestak in the primary, they also show Mr. Specter carrying high negative ratings and running in a dead heat with Mr. Toomey in the general election.

Even more than six months away from the primary and a year from the general election, all three of the major candidates are raising funds and campaigning at a brisk pace. Mr. Specter has raised $12.7 million this year; he said he needs $30 million to win.

Mr. Sestak was pressured by the national party and the Obama administration not to run against Mr. Specter, but the incumbent said he's not afraid of a primary challenge.

Mr. Leahy, who has known Mr. Specter since they were both prosecutors, said he hasn't seen much of a change in Mr. Specter in the past six months.

"He keeps on working just as hard as ever -- this guy is a workaholic," Mr. Leahy said. "He's in a tough campaign, he's running back and forth."

At 79, Mr. Specter, a two-time cancer survivor, is losing his hair and his gait is slow. But he said he feels about 37 years old and joked that the water in Pennsylvania keeps him and Penn State football coach Joe Paterno young. He plays squash or lifts weights every morning.

He says he has much left to accomplish: continuing funding for the National Institutes of Health, vetting Supreme Court nominees and working to get the Court televised.

When asked if he would serve a full six-year term if elected, Mr. Specter shot back with a question of his own: "Why limit it to a full term?"

Washington correspondent Daniel Malloy writes the "Pittsburgh On The Potomac" blog exclusively at PG+, a members-only web site of the Pittsburgh Post-Gazette. Our introduction to PG+ gives you all the details.