Sen. Arlen Specter (Pa.) should not expect dinner invitations from his GOP colleagues anytime soon, but Republicans said it is unlikely his decision to bolt from their party will have long-lasting effects on their relationships with the Pennsylvanian.

Specter’s return to the Senate floor Wednesday evening — where he voted against the Democratic budget resolution — was marked by a warm greeting from several top Republicans, including Minority Whip Jon Kyl (Ariz.) and Sen. Judd Gregg (N.H.).

But that doesn’t mean his transition will be smooth sailing: His switch to the Democratic Party will likely require Specter to cull some of his staff and return millions in campaign contributions from Republicans, and it could prompt fights over committee ratios.

Sen. John Thune (R-S.D.) said that while his defection hurts, Specter has developed a number of close friendships over his nearly 30 years in the Senate and it was unlikely those relationships would be affected.

“He has a lot of friends up here that have been [friends] for a long time, and I don’t sense that’s going to change,— Thune said.

Eric Ueland, a former Senate GOP leadership aide, said party switchers in the Senate are often more accepted by the losing side than one might assume, because of the historically collegial nature of the chamber.

“Out on the floor, there isn’t a lot of shunning that goes on,— said Ueland, who worked for several Senate GOP leaders including former Majority Leader Bill Frist (Tenn.).

Ueland acknowledged that switching does cause “old relationships to wither some and new relationships to flourish, but it’s not as if there’s some sort of back-turning that goes on.—

Thune also noted that Specter’s relationship with Republicans is likely helped by the fact that his departure will not prompt the tectonic shift in power caused by the 2001 defection of Sen. Jim Jeffords (I-Vt.).

“This doesn’t have the kind of game-changing quality. … On the Richter scale it doesn’t even come close,— Thune said.

Jeffords, who for years sided with Democrats on many partisan issues, turned control of the chamber over to Democrats and then-Minority Leader Tom Daschle (D-S.D.) when he abandoned the GOP.

Republicans — including Specter — took Jeffords’ decision hard. In a speech following Jeffords’ defection, Specter likened it to a death in the family, a sentiment that many Republicans shared. Jeffords, at least initially, found himself on the outs with many of his Republican colleagues who were deeply wounded by his decision and their sudden loss of the majority.

Jeffords took over the Environment and Public Works chairmanship as a direct result of his party change. But he never appeared able to fully repair many of his relationships with Republicans. As time wore on — and particularly after Republicans regained control of the chamber in the 2002 elections — Jeffords often seemed to have difficulty fitting in with either party.

Former Sen. Trent Lott (R-Miss.), who was Majority Leader at the time of Jeffords’ historic switch, agreed that Specter’s party change won’t rise to the same level, and he predicted that any wounds will heal.

“It will soften over time. It’s human nature,— Lott said.

Some Republicans, however, argued that it is much less clear whether Specter will enjoy the kind of easy transition into a new party that others have in the past.

For instance, Sen. Richard Shelby (R-Ala.) switched parties on Nov. 9, 1994, the day after Republicans wrested control of the House and Senate from Democrats. Former Sen. Ben Nighthorse Campbell (R-Colo.) abandoned the Democratic Party a few months later in March 1995.

Both Shelby and Campbell, despite some initial anger from Democrats, went on to have long and productive careers and over time developed new relationships with some of their former colleagues. Shelby and Campbell also found a comfortable home in their new parties.

Republicans warn that given his strong independent streak and an often cantankerous nature, Specter could find it much more difficult to fit in with his new colleagues.

One senior GOP aide said that over his 30 years in the Senate, Specter’s colleagues in the GOP have learned how to work with Specter and built strong working relationships with him, something that Republicans said is a time-intensive process.

“It had gotten to the point where everyone in our Conference interacted smoothly with Specter and understood that he could be cantankerous,— the aide said, warning that it will take time for Democrats to develop that kind of rapport.

“Six months or eight months down the line, no one’s going to care that he switched parties. They’re all going to see a D’ after his name and wonder why he keeps opposing things and being a problem,— the aide predicted.

Specter’s decision could prompt a potentially ugly fight between Majority Leader Harry Reid (D-Nev.) and Minority Leader Mitch McConnell (R-Ky.) over the makeup of the Judiciary and Appropriations committees, on which Specter serves. Unlike Jeffords, who became chairman of the Environment Committee because he helped Democrats take control of the Senate, Reid has already said Specter’s switch will not mean a change in chairmanships, even though he will retain his seniority and committee assignments.

But because the chamber’s organizing resolution sets out specific ratios for Democrats and Republicans on each committee, simply shifting Specter over to the Democratic side would technically violate those rules. Republican leadership aides have said that because Specter serves on two “A— committees, some adjustment will be needed — either by having one Democrat step down or adding a Republican to the committees’ rolls.

But Democratic leadership aides have said Reid is not contemplating further changes to the committee structures at this point. One Democratic source familiar with the situation hinted that if Republicans push the issue, Democrats could call for a complete restructuring of the organizing resolution to reflect their expanded majorities — particularly if Al Franken is declared the winner of his race against former Sen. Norm Coleman (R-Minn.).

“The organizing resolution was based on 58-42 margin, not 60-40. So if we open up new negotiations, then it would have to factor a larger number for Democrats and fewer for Republicans,— this Democrat said.

And then there is the question of what Specter will do with his committee staff. All told, some 34 staffers worked for Specter on the Judiciary and Appropriations committees, and it is unclear whether he will be able to retain some of those employees, or if Republicans will decide to retain them.

Lott said he believes Specter will ultimately regret his decision, and he predicted tough times ahead for his former Pennsylvania colleague, both in his re-election and in the Senate.

“I don’t know what will happen with Arlen. I wouldn’t be surprised to see him lose in the Democratic primary,— Lott said. “Certainly, the Democrats are getting a boost, and maximizing on this, and I can’t blame them at all. But I don’t think it’s going to work out great for him, that would be my guess.—

Lott, who said he was planning on doing a fundraiser for Specter as of Tuesday morning, said while he doesn’t “wish him ill,— he fears the worst: “There’s a possibility of him not winning [re-election], and just going forward, he needs to be careful with this type of thing. He may find that he’s not a Republican, and he’s not a Democrat. He could become sort of a man without a party.—

But Democrats predicted that Specter’s situation is more analogous to the Democrats who switched to the GOP following the Republican Party’s stunning wins in 1994. Unlike Jeffords, whose switch delivered control of the Senate to Democrats, Specter is just making the Democrats one man stronger.

“Jeffords was regarded as a hero by the Democrats, because he made half of the caucus committee chairs,— said one former aide to Daschle.

The aide added that Specter’s switch “is a psychological blow to the Republican Party and a psychological win for the Democratic Party, but substantively, it doesn’t change anything.—

Specter’s long history in the chamber suggests that he will continue to cause heartburn for the majority on tough votes.

“Unless you are a chairman, you are only relevant if people don’t know how you’re going to vote,— said the Daschle aide.

However, one Senate Democratic leadership aide said Specter now has more incentive to tack to the left on votes in order to solidify support within the Democratic Party for his re-election.

“He may be a more reliable vote than some of the Democrats we have now,— said the aide. “Sen. Specter made a calculation that the only way he’s going to win [in 2010] is to run as a Democrat, so you have to assume he’s going to vote with Democrats as much as possible.—

As for how Democrats who have been burned by Specter might react to him joining their ranks, the Daschle aide suggested that “past sins are forgiven— in the initial welcoming.

Erin P. Billings contributed to this report.