Arlen Specter is home. And he’s comfortable.

Nearly one year after Pennsylvania’s senior Senator left the Republican Party and became a Democrat, the move makes so much sense to his colleagues on Capitol Hill — both Republican and Democrat — that no one seems to remember what the uproar was all about.

Specter, 80 and winding down his fifth Senate term, announced plans to switch parties on April 28. The move had broad political implications in Pennsylvania, where Specter is running for re-election. In the Senate, Specter’s change of allegiances delivered to the GOP what was at the time yet another body blow, while providing the Democrats with a 60-vote, filibuster-proof majority.

But the dust settled rather quickly — at least in Washington, D.C. — and it was as if Specter’s 40 years in the Republican Party were an error of circumstance that was belatedly rectified.

“I think Arlen is more comfortable on the Democrat side. But he’s doing fine. We’re really good friends. If I interpreted right, he’s more comfortable,” said Sen. Orrin Hatch (R-Utah), who has been close with Specter since he arrived in the Senate in 1981 and has served on the Judiciary Committee with him for years. “Arlen is not a conservative anyway. He’s more of a moderate liberal, and was that as a Republican. But he’s a good person, and I have a high regard for him.”

“Comfortable” is the word Specter used Thursday to describe his time as a member of the Democratic Conference. In fact, while his positions on social and some legal issues often put him at odds with his Conference when he was a Republican, as a Democrat Specter has been one of Majority Whip Dick Durbin’s (D-Ill.) most reliable and loyal votes.

Still, as Specter made abundantly clear last April, his departure from the GOP was a political decision that had less to do with philosophical comfort than with his prospects for winning a sixth term. At the time, the Senator’s internal polling showed he would lose a GOP primary rematch with former Rep. Pat Toomey, and Specter said he was unwilling to allow Pennsylvania Republicans to have the final say on his Senate career.

[IMGCAP(1)]Specter’s party switch enabled him to avoid a primary against Toomey, whom he only very narrowly defeated in 2004 after receiving high-profile assistance from President George W. Bush and then-Sen. Rick Santorum (R-Pa.). But after receiving an endorsement for re-election from his new party leader, President Barack Obama, Specter ended up smack in the middle of a potentially competitive Democratic primary.

Rep. Joe Sestak’s (Pa.) decision to challenge Specter from the left, combined with the Senator’s need to prove his bona fides to his new party, could explain why he’s no longer a swing vote and the center of attention on major issues before the Senate. Specter said his positions over the course of his career have been consistent but that the context has changed.

“My record, as evidenced from the voting, has been one of independence, not ideological, and I have, I think, on the big issues voted with the Democrats as often — perhaps more often than with the Republicans,” Specter said during an interview in his spacious Capitol Hill office. “When I voted for increasing the minimum wage and extending unemployment compensation as a Republican, it stood out like a sore thumb. When I vote that way as a Democrat, nobody notices.”

As Pennsylvania moved more toward the Democratic Party over the past 10 years, the state’s GOP base contracted and moved further to the right. In suburban Philadelphia, Specter’s base, the Republican Party is particularly weak. The Democrats in recent years captured a handful of Republican Congressional districts, the governor’s mansion, and in 2006 Sen. Bob Casey (D) ousted Santorum, winning by 18 points.

Specter’s decision to leave the GOP only solidified a growing disaffection for him at home among Republican voters while alienating longtime Republican backers who stood by him despite his moderate and liberal positions on key issues. But in Washington, D.C., Specter still has friends in the party, including his former fellow GOP Senators and Republicans who work downtown.

One Republican lobbyist with Pennsylvania ties said the secret to Specter’s success is his work ethic and doggedness and the top-notch constituent services delivered by his office. This lobbyist, still a Specter fan, said the Senator’s views have always been to the left of the GOP mainstream but that he managed to win statewide partly because he forged relationships with Democrats as well.

Specter lost Senate and gubernatorial GOP primaries in 1976 and 1978, respectively, before winning a close Senate primary in 1980 and riding the Ronald Reagan wave into office. Specter’s career since then was unorthodox for a Republican: He had close ties to organized labor, and as an ardent proponent of abortion rights was well-liked by feminist groups.

Since becoming a Democrat, Specter has moved beyond Democratic interest groups and reached out to grass-roots Democrats in all 67 Pennsylvania counties. Casey, whose family is something of Democratic royalty in the Keystone State, said Specter has made enormous progress on this front while making himself at home among Senate Democrats on Capitol Hill.

“I think what he did from that day forward is really work to earn — and is still working — to earn the trust of Democrats. That meant a lot of extra work, a lot of extra travel, meeting people that he may have known a little bit but never really interacted with,” Casey said. “But I was especially impressed at what he did and continues to do at the grass-roots level, meeting people literally county by county.”

“He’s been just wonderful to work with,” Durbin added. “He’s been an active member of our caucus. I mean, there was not much of a learning curve there. Arlen fit right in.”

Switching parties cost Specter his ranking position on Judiciary and a chance to be at the forefront when Obama nominated now-Supreme Court Justice Sonia Sotomayor. Specter was sent to the back of the seniority line among committee Democrats, although the Senator said he expects that issue to be revisited.

Specter said he doesn’t miss the limelight that came with his former status as a moderate Republican, although he didn’t sound overly convincing, hesitating for a moment before finally settling on a simple “no” when asked.

Specter, a former GOP Judiciary chairman who a few years back helped shepherd Bush’s two Supreme Court nominees across a Democratic minefield, insisted he was just as effective during the Sotomayor confirmation hearings as he might have been as a senior member of the minority.

Specter’s work product and approach to vetting judicial nominees has not changed since he switched parties, said both Republicans and Democrats who serve on Judiciary, Chairman Patrick Leahy (D-Vt.) included.

“The seniority issue’s not over yet. There’s more coming on that,” Specter said. “But at the time [of the Sotomayor hearings], I spontaneously said, ‘It’s not a matter of where you sit, it’s a matter of what you say.’ I played a pretty important role when I was sitting way down the line” during nomination hearings in years past, “and I had a pretty prominent role in Sotomayor even though I was way down the line.”