Your basic Pennsylvania couch potato, having gorged by now on a surfeit of campaign commercials, may well believe that the heavy hitters in both parties adore Arlen Specter.

In the various TV ads, George W. Bush lauds the loyalty of Arlen Specter ("I can count on this man"), Barack Obama says he loves Arlen Specter, and Rick Santorum is shown applauding Arlen Specter. There's a freeze-frame shot of Sarah Palin sharing a podium with ally Arlen Specter, but we also know that Ed Rendell and the labor unions are working to stoke turnout in Tuesday's Senate Democratic primary for the benefit of Arlen Specter.

All of which prompts me to wonder, and not for the first time: Who exactly is Arlen Specter, anyway?

Thankfully, the primary results will provide some minimal clarification; he'll be a winner, or a loser. If he squelches the serious challenge from upstart Congressman Joe Sestak, the story line will be that Specter's sudden morphing from Republican to Democrat was a shrewd move; that this tough old bird could probably outlive the cockroaches during a nuclear winter. But if he is ousted by the Democratic primary voters - a very real possibility; one late poll had Sestak up by nine points - it shall be written that not even Specter could survive the undertow that threatens to drown incumbents in both parties.

Two Capitol Hill insiders have been summarily dumped by their respective parties during the last eight days: 18-year Republican Sen. Bob Bennett (who couldn't even get renominated at his Utah party convention), and 28-year Democratic congressman Alan Mollohan (whose well-earned reputation for pumping pork into his West Virginia district failed to impress the primary voters). Anger is the emotion of choice in 2010, and career politicians make the best targets.

Specter being Specter, his vulnerabilities are unique. He has logged 30 years in the Senate, 29 of them as a Republican - yet his fate now hinges on convincing millions of Pennsylvania Democrats (many of whom have consistently voted against him since 1980) that he's really one of them. He hasn't been able to close the sale, and all that Bush-Santorum-Palin footage aired by Sestak at the eleventh hour hasn't made the job any easier. Indeed, there's more footage in Sestak's Internet ads, with Santorum lauding Specter for his yes votes on the original Bush tax cuts.

Joe Sestak is hardly a perfect Democratic alternative. His reputation as an extreme taskmaster is apparently well-earned, his support for an expanded war in Afghanistan has given some liberals pause, and he needlessly slowed his own progress several months ago when he claimed (before quickly clamming up) that the Obama White House had offered him a cushy federal job in exchange for his agreeing to quit the race. But the race is really a referendum on Specter, a career politician who has played so many sides of the fence that it's virtually impossible to situate him.

He has seemingly been everywhere, which arguably leaves him nowhere. He says he voted for Bush-Cheney and McCain-Palin (lauding Palin in '08 as "sort of a commonsense people person"), but says he'll vote for Obama in '12. He voted against Elena Kagan for solicitor general, but says he has "an open mind" about her ascent to the Supreme Court because "it is a distinctly different position." He voted against Robert Bork for the high court, but famously defended Clarence Thomas and voted for John G. Roberts Jr. and Samuel A. Alito Jr., although now, with respect to Roberts, he says that he made an error in judgment. He backed the economic stimulus while still a Republican, and has since moved leftward on health care and labor law, but one year ago, on Meet the Press, he cautioned: "I did not say I would be a loyal Democrat."

It was once written, about the 19th-century British statesman William Gladstone, that he was "a man of conviction, but this did not preclude changes in his convictions; bit by bit, he entirely changed his political creed." I suppose that's a kind way of saying that, for some politicians, the first principle is flexibility. Specter fits that category, perhaps never more so than in his memorable straddle of 1999, when he decided that Bill Clinton was neither guilty nor innocent in the impeachment trial; rather, he riled both parties when he declared the case "not proven," a middle-ground verdict that is legal across the pond in Scotland.

So the intangibles would seem to favor Sestak on Tuesday; the two-term suburban Philadelphia congressman is a relatively fresh face, whereas, given the current anti-incumbent environment, Specter may well have worn out his extended welcome. Even when Specter was enjoying double-digit leads over Sestak in the early polls (largely because Sestak was little known statewide), more than half of Pennsylvanians were saying that Specter didn't deserve a sixth term. And another key polling statistic stood out: Among those who were familiar with both candidates, Sestak was the clear favorite.

It's possible, of course, that organization will trump the intangibles, that the Rendell-labor-establishment machinery will pull Specter across the finish line. Whatever happens Tuesday, Democrats will need to heal their wounds before taking on Republican Pat Toomey in the fall finale. Bad feeling abounds. Many in the Sestak camp privately view Specter as a crotchety weather vane who needs to be consigned to a place with shuffleboard and Bingo. Many in the Specter camp privately view Sestak as an ungrateful upstart whose refusal to wait his turn compelled Specter to spend a lot of money that should've been banked for the autumn battle.

Specter's assumption, all along, was that he'd be the stronger autumn candidate; as he remarked May 1, in his sole debate with Sestak, "A very overarching issue in how you select a Democratic nominee is who can beat Toomey." But the latest polls show that Sestak - lacking Specter's baggage - would be more competitive in the fall. If the large pool of undecided Democratic voters tilt Sestak's way on electability, the incumbent's fate may well be sealed.

Word has it that Specter, at age 80, still plays a fiercely competitive game of squash. In the macho environs of The Sporting Club in Philadelphia, facing off against foes decades younger, he grunts and plays the angles, as always. But one gets the sense that, politically at least, the walls may be closing in.

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