Ramesh Ponnuru has a great find: Arlen Specter's indignant reaction to James Jeffords' 2001 party switch. An excerpt:

I intend to propose a rule change which would preclude a future recurrence of a Senator's change in parties, in midsession, organizing with the opposition, to cause the upheaval which is now resulting.

I take second place to no one on independence voting. But, it is my view that the organizational vote belongs to the party which supported the election of a particular Senator. I believe that is the expectation. And certainly it has been a very abrupt party change, although they have occurred in the past with only minor ripples, none have caused the major dislocation which this one has.

When I first ran in 1980, Congressman Bud Shuster sponsored a fundraiser for me in Altoona where Congressman Jack Kemp was the principal speaker. When some questions were raised as to my political philosophy, Congressman Shuster said my most important vote would be the organizational vote. From that day to this, I have believed that the organizational vote belonged to the party which supported my election. ...

Accepting Senator Jeffords ' decision was based on principle for the reasons he gave at his news conference on Thursday morning, a question still remains as to whether any such inducement was offered and whether it played any part in Senator JEFFORDS' decision. Questions on such offers and counteroffers should be considered by Senators and by the Senate in an ethical context, but at this moment I do not see any way to effect such conduct by rulemaking or legislation.

Meanwhile, Chris Bowers remembers this 2004 ad for Specter, starring George W. Bush:

I think the Democrats have a problem here. They obviously want the Specter switch to go well -- if nothing else, to maintain their credibility when inducing future party switches. But there is a wide-open opportunity for whoever challenges Specter in the Democratic primary. Commercials like the one above are not going to play well with a Democratic primary electorate, and it's no doubt just the tip of the iceberg of opposition research that's going to pop up.

Specter and the Democratic leadership understand that there's a lot of cynicism in politics. Specter said and did a lot of things he didn't believe in as a Republican in service of what he saw as a larger end -- and which also served his career -- and now he's going to do the same as a Democrat. But that reality is something that politicians don't share with the voters. You need some kind of public conceit to maintain the appearance of principle. It's going to be much harder for Specter to pull this off than people think. Political elites are underrating the difficulty of the task because they've assimilated the phoniness and moral trade-offs in politics to a degree that the general public hasn't. Voters may think that Congressmen in general are dishonest, but they like to think that their member of Congress is a stand-up guy. Specter may have all the money and endorsements in 2010, but he'll have huge vulnerabilities on both character and ideology.

--Jonathan Chait