After three decades in the U.S. Senate, Arlen Specter's high-wire act finally may be reaching an end -- with what looks to be a nasty tumble.

Whether the helpful shove comes courtesy of Republicans or Democrats, it will make for fine spectator sport -- since the Pennsylvania senator has made a career of antagonizing both parties in equal measure.

But even though he's given the GOP plenty of reasons for dissatisfaction throughout his tenure, party stalwarts at both the state and national level should hesitate before they give him a push. After all, for the badly outnumbered minority party, half a Republican is better than none.

On the right, his vote against ending the Republican filibuster of the unions' so-called "Employee Free Choice Act" is dismissed as a cynical ploy to defuse a 2010 primary challenge from conservatives furious at his support for President Barack Obama's pork-laden stimulus bill.

And on the left, this anti-union vote is a gimme for Democrats ready to take him on in a state where they now enjoy a post-Obama voter registration edge of 1.2 million. Ed Rendell exits the governor's mansion in 2010, just in time to move on to the U.S. Senate, while the party's state chairman reports that some Democrats are actively courting Franco Harris, the former Pittsburgh Steelers great.

At this point it seems the only thing that could salvage Mr. Specter's chances is the possibility that the Obama administration's enormous economic gambles fail to pay off, further damage the nation and thus tarnish the Democratic brand.

Mr. Specter's identification with the GOP brand was rocky from the get-go. He turned to the party only when he failed to win the Democratic nomination for Philadelphia's district attorney race in 1965. He changed his registration to Republican only after he won.

When he made it to the U.S. Senate on his second try, in 1980, he pooh-poohed the idea that he owed his victory to Ronald Reagan's coattails. Seven years later, his lack of support essentially sank the nomination of Robert Bork to the U.S. Supreme Court.

But disgruntled Republicans got some satisfaction five years later by using Mr. Specter's sizable ego against him during the Clarence Thomas hearings. Fearing he might "bork" Mr. Thomas, Republicans asked him to lead the interrogation of Anita Hill, who'd accused Mr. Thomas of sexual harassment; they figured he'd go for broke, playing prosecutor before a huge television audience, and would thus be unable to vote against the nominee.

Mr. Specter obliged -- or fell for the gambit, perhaps, enraged ardent feminists nationwide, but survived a tough challenge from Lynn Yeakel (51 percent to 48 percent).

For philosophical conservatives, the irreconcilable difference between his positions in the Bork and Thomas hearings settled early on the question of whether the senator acts primarily according to principle or in political self-interest. Observers long ago noted his tendency to indulge his liberal leanings for the first few years of each term, then to lean right for two years before each primary -- an explanation that fits the timing of his Bork/Thomas votes (1987 and 1991, respectively).

But every once in a while, Mr. Specter takes a stand that defies this re-election agenda. Recall his odd contortions during President Bill Clinton's impeachment trial -- citing Scottish law and voting "not proven." He had just handily defeated his Democratic opponent and had nothing to fear from a yes-or-no vote. If there's any overarching philosophy at work here, it's known only to Mr. Specter and a few law professors in Edinburgh.

Such zigzags put his vote on "Employee Free Choice Act/Card Check" in a different light. In announcing his latest controversial decision, Mr. Specter deplored the intimidation both sides employ in union-organizing efforts and of the need to strengthen collective bargaining without jettisoning the sacred privacy of the vote.

Indeed, human nature being consistent from the corporate suite to the loading dock, a secret ballot is the only way to ensure that a worker is truly voting as his conscience leads, not as managers or peers dictate. Mr. Specter invoked the same independence of conscience in making his decision. Maybe this time, that's really his motivation.

Mr. Specter will be 80 years old in 2010. He has survived open-heart surgery and two bouts of cancer. Perhaps he's looked at Pennsylvania's increasingly Democratic landscape and decided to end his career his way.

If so, conservatives should stop spluttering long enough to thank him for taking an unpopular but principled stand on "(Not Very) Free Choice." For the same reason, they should think twice before pushing him toward an involuntary retirement.