No one knows why the chicken crossed the road, but why Arlen Specter crossed the aisle to the Democrats is a matter of rather less mystery, if intense debate. The why is quite simple: Free-range Republican, he was about to lose next year's Pennsylvania primary by a large margin, a problem he solved by changing his voter pool. But it was the how that is rather more pertinent: He was under attack from conservatives in his own party, opening questions less of whether the good and the best can be enemies than of whether removing the mediocre turns out to be worth it when the result is the worst of all worlds.

Specter was targeted in the primary by a conservative with the backing of the free-market, low-tax Club for Growth, pegged by National Review's Ramesh Ponnuru as "The Club for Shrinkage," for its record of going after imperfect incumbents with down-the-line pure conservative activists, who go on to lose in the fall to a Democrat. In the Washington Examiner, Timothy Carney suggested that Specter's defection was prompted by the intention of Senator Jim DeMint of South Carolina to back Specter's challenger, which DeMint had confided to Specter about five days before Specter switched. DeMint expressed no regrets: "I would rather have 30 Republicans in the Senate who really believe in principles of limited government, free markets, free people, than to have 60 that don't have a set of beliefs," he said. The fact that 30 senators cannot make policy didn't

DeMint is far from alone. After the 2008 election was over, an election which saw Republican numbers grow ever more minuscule, activists on all sides began vigorous efforts to make the party still smaller by purging from it everyone who failed to act, think, and talk as they did themselves. On op-ed pages, in magazines, on websites and cable and radio, a wide range of Republicans and right-wingers announced themselves shocked, appalled, mortified, and repelled to the point of nausea to find themselves in the same party with the likes of David Brooks, David Frum, Sarah Palin, Rush Limbaugh, Meghan McCain, Karl Rove, and a collection of other politicos, pundits, and personalities whose presence they seemed to find intolerable. A passion for coalition-destruction seemed to rage on all sides.

What lies behind this is (a) the feeling that oneself and one's friends make up a majority; and (b) a failure to realize that a party and movement are not the same thing. A movement exists to express and promote a coherent set of principles in the world of ideas and of values. A party--especially in a two-party system--is something quite different: a gathering of diverse political forces around a large and loosely held set of interests and values, that exists to give all of its factions access to power in the practical world of events. A movement gives a party a spine and a platform; the party assembles a coalition around them that is large enough to win and hold power, and turn some of the movement's ideas into law.