Sen. Joe Lieberman’s (ID-Conn.) endorsement of Republican Sen. John McCain (Ariz.) for president earlier this week rattled some of his Democratic colleagues, one of whom said he was “speechless” after his conference mate announced his choice.

“Everyone has the right to make their own decisions,” Sen. Byron Dorgan (D-N.D.) told Roll Call on Monday. “I wish he hadn’t done what he did, but he’s a friend of mine.”

Despite Lieberman’s defection, Congressional insiders believe there is little chance that the Independent Senator who caucuses with Democrats will cast his lot with the other side anytime soon.

“Joe has voted with us on most domestic issues,” said Sen. Charles Schumer (D-N.Y.).

But they don’t expect him to switch back and become a Democrat, either.

First off, experts agree that constituents frequently hate party switching. Ex-Rep. Michael Forbes (D-N.Y.) lost his job to an unknown librarian in the 2000 Democratic primary a year after leaving the GOP.

If he became a Republican or formally rejoined the Democrats, Lieberman would lose his considerable leverage with both parties, nixing his ability to let his political juices flow — and attract the considerable media attention he gets when he defies party leaders on both sides.

But even more, House and Senate experts say that party switching is a rarity in Congress’ 200-plus year history, a trend that is obscured for many modern-day Congress watchers by the rash of aisle crossing that occurred in the mid and late 1990s in the wake of the Republican takeover of both chambers.

Congressional historians claim that less than 2 percent of both chambers’ memberships have switched parties since the 1st Congress was gaveled open on March 4, 1789. And with partisanship in Congress possibly running at an all-time high, the chances of a party switch today almost are nonexistent.

“In an American Congress, it’s very rare — especially during times of stable, party competition,” said Tim Nokken, a political science professor at Texas Tech who specializes in party switching. “Even [after 1994] it was only five Members of the House and two in the Senate.”

“And it’s probably a good thing we don’t have a lot of party switching,” Nokken continued. “Because as weak and unguided as we see our parties, they are significant enough to keep their membership pretty steady and politicians don’t completely flip-flop from one party to the next.”

Between November 1994 and December 1995, six Southern Democrats and one moderate Colorado Senator joined the new Republican House and Senate majorities on their trip out of the political wilderness: Rep. Nathan Deal (Ga.), Sen. Richard Shelby (Ala.), then-Reps. James Hayes (La.), Mike Parker (Miss.), Billy Tauzin (La.) and Greg Laughlin (Texas) and then-Sen. Ben Nighthorse Campbell (Colo.).

Since then, fellow Southern Reps. Rodney Alexander (La.), Ralph Hall (Texas) and Virgil Goode (Va.) have switched parties. One-time Independent Rep. Jo Ann Emerson (Mo.) also became a registered Republican in 1997, retired Sen. Jim Jeffords (I-Vt.) went the other way and ex-New Hampshire Sen. Bob Smith switched twice between Republican and Independent during a four-month period eight years ago.

Smith settled for good with the GOP in November 1999 — but he wound up losing the Republican primary when he ran for re-election in 2002.

Sarah Binder, a Congressional expert at The George Washington University and a Brookings Institution fellow, said that for many lawmakers, party affiliation is sacred ground and most Members would consider it high treason to switch, even if voters and the media are less wedded to one political party or another.

“We shouldn’t be surprised that there doesn’t seem to be any party switching afoot,” Binder said. “Partisanship is a pretty stable thing, especially for Members of Congress, who [now] come from more activist ends of their parties.”

Binder and others also attributed the lack of aisle crossers — particularly in the House – to the dearth of moderates in the chamber. A decade ago, the Southern Democratic exodus to the GOP was more administrative than ideological. Most of their constituents already had supported Republican presidential candidates going back to the 1960s.

Political heritage, combined with the lack of a well-oiled Southern GOP machine, drove the Democratic turnout, historians argue. Southern Democrats had grumbled about being out of step with party leaders for decades before ex-Speaker Newt Gingrich (R-Ga.) stormed to power in 1994.

“They were just catching up with their constituencies; they had been thinking about it and talking about it for years … going back to the late Sen. Strom Thurmond (R-S.C.),” Michael Bailey, a government professor at Georgetown, said. “In those Southern districts, there was more of a one-party rule attitude and it took them awhile to go from one-party Democratic, to one-party Republican, rule.”

He continued: “It came on pretty fast [in 2006], so that change hadn’t been brewing for so long.”

Binder said one place Democrats may have looked for potential moderate GOP defectors was in the Northeastern states of Connecticut, Rhode Island, New Hampshire and New York — had former Democratic Congressional Campaign Committee Chairman Rahm Emanuel (Ill.) not gutted their ranks in last year’s elections.

Freshman House Democrats now control seats formerly occupied by moderate Republicans such as Reps. Charlie Bass (N.H.), Jeb Bradley (N.H.), Mike Fitzpatrick (Pa.) Gil Gutknecht (Minn.), Melissa Hart (Pa.), Nancy Johnson (Conn.), Sue Kelly (N.Y.) and Jim Leach (Iowa) and Sen. Lincoln Chafee (R.I.). Until his defeat last year, Chafee frequently was the object of rumors that he might switch parties.

“The corollary for the Republican Party to Southern Democratic dominance is the Northeast,” Binder said. “But as those states have turned Democratic at the presidential level, it’s much tougher for those moderate Republicans to win.

“The fact that there are so few of them left — Sens. Arlen Specter (R-Pa.), Susan Collins (R-Maine), Olympia Snowe (R-Maine),” she added. “That’s quite a difference of what you would have seen 30 years ago.”

But should Democrats increase their majority in the House next year, Nokken said, rare moderates such as Rep. Christopher Shays (R-Conn.) undoubtedly may consider the option more seriously than they did in the early months of 2007.

“There are some Republicans left like Shays, he’d be an individual most likely to switch,” Nokken said. “In the past it was conservative Democrats who could gravitate to the Republican Party and not change that much.”

Nokken added: “Shays wouldn’t have to change that much to fit in [with Democrats].”

Erin P. Billings and Emily Pierce contributed to this report.