Sen. Arlen Specter of Pennsylvania has a reputation as something of a political chameleon. In leaving the Republican party Tuesday to join the Democrats, he was reversing his move to the GOP made more than 40 years ago.
Then again, he may have been pushed. His switch is representative of a broader trend that is redefining and realigning American politics. And it is there that it might have the greatest long-term repercussions.
Specter's defection matters for several reasons. Chief among them is that with Al Franken seen as the probable victor in the still-contested Minnesota Senate race, the addition of Specter would boost the ranks of Senate Democrats to 60 - the number of votes needed to invoke cloture and override a filibuster.
Specter was emphatic in his announcement that he would not be "a party-line voter any more for the Democrats than I have been for the Republicans." He was one of only three Republicans in the Senate to support President Obama's economic stimulus plan.
Specter also pointed out that he strongly differs with his new party on a number of issues. The "card-check" provision that would no longer require a secret ballot in votes to unionize a company is one example.
And in that sense, Specter could help his new party stay focused on the middle. Hard-core conservatives have dismissed him as Rush Limbaugh did saying, "It's ultimately good. You're weeding out people who aren't really Republicans."
Except that his record is hardly that of a Moveon.org activist. And as such he could help tip some Democratic positions ever so slightly to the right and perhaps keep that party more focused on the middle.
Specter's choice to change parties was apparently based at least in large part on a personal calculation that he was unlikely to survive a primary challenge from former U.S. Rep. Pat Toomey. He almost beat Specter once before and had been up 10 points in the polls.
Specter is a five-term senator, popular with general-election voters. But, as Specter pointed out, the ideologues in the Republican Party have been systematically eliminating the moderates in their ranks, and in doing so they have lost seats in Congress and influence in Washington.
Examples include former Reps. Wayne Gilchrest of Maryland, Joe Schwarz of Michigan, New Mexico's Heather Wilson and former Sen. Lincoln Chafee of Rhode Island. All were incumbents who lost primary races to more conservative Republicans who then went on to lose in the general election. The prevalence of this phenomenon - and the public endorsement of it by the likes of Limbaugh - suggests that it is accepted and even supported by the party's leadership. Why?
A look at the electoral map shows the result. Outside of Appalachia and the South, in the 2008 election the GOP carried solidly Republican Utah, John McCain's home state of Arizona and a block consisting of Nebraska, the Dakotas, Wyoming, Montana and Idaho - all six of which add up to fewer electoral votes than Florida, which Obama won.
The Republican Party is reducing itself to a regional party representing and dominated by the Deep South. And by alienating remaining moderates like Arlen Specter it is advancing the process.
That is not healthy for either party or the American people. Ideological purity may satisfy zealots but it does little for democracy.