It's been more than four decades since Arlen Specter, the senator from Pennsylvania, earned the nickname "Specter the Defector." This week, he confirmed that it is indeed an accurate description of his political character.
I was a kid reporter for The New York Times back in 1965, when Specter's flip-flopping first attracted attention, and the report I filed recounts the circumstances that led to his unflattering nickname.
Specter, then a Democrat, had been an assistant district attorney in Philadelphia, and he harbored an ambition to run against his lackluster boss, a man named James Crumlish. The Democratic bosses of Philadelphia were not encouraging Specter because, as one of them told me, "We don't want another young Tom Dewey," the reform-minded New York prosecutor who launched himself into the governorship and two presidential nominations by sending a string of prominent officeholders of both parties off to jail.
So Specter, with the encouragement of such prominent Pennsylvania Republicans as Sen. Hugh Scott and Gov. William Scranton, said he would run against Crumlish on the GOP ticket. To hedge his bets, and to help himself gain Democratic votes, he waited until he won that race to change his own party registration.
Over the decades since, Specter has become one of the senior Republican senators and the best Republican vote-getter in Pennsylvania. But his frequent defections from GOP orthodoxy, not just on abortion but on labor issues, taxes and spending, have made him vulnerable to challenge in the state's Republican primary.
Former Rep. Pat Toomey, a right-wing ideologue, came close to upsetting Specter in the 2004 Senate primary, and next year, Toomey looked to be a better than even money bet to knock off the incumbent.
At one level, Specter's decision is symptomatic of the narrowing of the GOP spectrum, a sign of the increasing dominance of that shrunken party by its most conservative, Southern-accented members. There are no Republican House members left in New England. A traditionally Republican House seat in upstate New York has just flipped to the Democrats, and both coasts, the Southwest and the upper Midwest are increasingly voting for Democrats.
That is why Republicans have lost their majority and their veto power over legislation in the House and why they may soon lose the ability to filibuster and delay Democratic measures in the Senate, when Specter switches and Al Franken finally claims the Minnesota seat.
But as much as Specter's decision reflects an increasingly serious weakness in the Republican Party, there is no escaping the fact that it is also an opportunistic move by one of the most opportunistic politicians of modern times.
The one consistency in the history of Arlen Specter has been his willingness to do whatever will best protect and advance the career of Arlen Specter.
When some Republicans in 2004 challenged in the GOP caucus his elevation to the chairmanship of the Judiciary Committee, Specter assured them that he would not use the post to block any Supreme Court nominees of President George W. Bush. And despite his sometimes liberal record, he voted for both Chief Justice John Roberts and Justice Samuel Alito.
Just a few weeks ago, when he was still calculating how he might survive a Republican primary against Toomey, he announced that - despite his friendship with labor - he would not support the so-called card check legislation that is the No. 1 priority of the unions.
This is the man who now claims the Democratic nomination in Pennsylvania.
Specter has been welcomed to the Democratic Party by President Barack Obama and by Pennsylvania Gov. Ed Rendell, the most influential Democrat in Harrisburg. That makes it unlikely that Specter will face any serious challenge in next year's Senate primary. And, if his health holds up, he will be a strong favorite against Toomey in the November election.
So once again, Specter is likely to reap the political reward from his maneuvering. But the Democrats should be open-eyed about what they are gaining from his return to his original political home.
Specter's history shouts the lesson that he will stick with you only as long as it serves his own interests - and not a day longer.
E-mail nationally syndicated columnist David Broder at email@example.com.