Long before Pennsylvania Senator Arlen Specter announced he was switching parties, his conservative colleagues began targeting him over his blasphemous support of the Obama stimulus package. Republican Chairman Michael Steele, who in February said he was "open" to punishing Specter, today called the new Democrat's about-face "not only disrespectful, but it's just downright rude." As it turns out, the venom Republicans have reserved for Arlen Specter is just more of the same poisonous retribution they wrought on Vermont's Jim Jeffords in 2001.
While Senate Republicans were stunned by Specter's announcement today, Jeffords' departure from the 50 member GOP caucus was in the making for weeks.
When the Vermont Republican bucked President Bush over his planned $1.6 trillion tax cut for the wealthy in 2001, Bush retaliated with the same vicious "politics of payback" that came to define his tenure in the White House. As I noted in 2004:

An early indication of the vindictiveness of this administration came with the saga of Vermont Senator Jim Jeffords' defection from the GOP in 2001. This is a tale of double-retribution. First, Jeffords refused to back the Bush tax cut plan in 2001. As The New Republic reported in June 2001, the White House responded by gutting special education programs supported by Jeffords and by threatening the Northeast Interstate Dairy Compact critical to the Vermont milk industry. To add insult to injury, the Bush team took the unprecedented step of not inviting Jeffords to a White House event honoring a teacher from Vermont. They even denied Jeffords' office White House tour passes for his constituents. His departure from the GOP seemed understandable then and now; his one-time colleagues of course are making his tenure as an independent a lonely one.

Like General Eric Shinseki, counter-terrorism czar Richard Clarke, Treasury Secretary Paul O'Neill, Medicare actuary Richard Foster and many more, Senator Jeffords paid the price for crossing George W. Bush and his Republican allies. When Jeffords in May 2001 declared himself an independent and temporarily turned control of a divided Senate over to the Democrats, Trent Lott (R-MS) called it "a coup of one" and groused, "There is only one person to blame for all this, and that's Jim Jeffords." Ironically, future lapsed Democrat Joe Lieberman responded to Jeffords' joining the Democratic caucus by announcing:

"This is historic. It gives us the opportunity to set the agenda."

Ultimately, Jeffords was shunned by his Republican brethren and was even booted from the "Singing Senators," a group of GOP colleagues which ironically included Idaho's Larry Craig. Ostracized and isolated by his former friends, Jeffords retired from the Senate in 2007.
As I suggested this morning, Arlen Specter's Hamlet-like performances on the U.S. attorneys purge, NSA domestic surveillance, presidential signing statements and the Employee Free Choice Act make him a dubious if still welcome addition to the ranks of Senate Democrats. (It is with good reason that The New Republic's Jonathan Chait mocked Specter as "Unprincipled Hack [D-PA].") But if Democrats are rightly skeptical, furious Republicans will be looking for payback.
Just ask Jim Jeffords.
UPDATE: As Politico and the Los Angeles Times each noted, Specter was among those Republicans who tried to dissuade Jeffords from leaving in 2001, going so far as to propose a rule change to prevent such mid-term party changes in the future.