IT TOOK MONTHS for staffers at Philadelphia's top political-consulting shop, the Campaign Group, to find exactly the right TV clip they wanted of Sen. Arlen Specter explaining why he was switching to the Democratic Party.

It took about five seconds for the firm's founder, local political-ad guru Neil Oxman, to take the spot - the cornerstone of Rep. Joe Sestak's successful campaign to oust Specter - and make it perfect.

It was as soon as Oxman heard the short clip of Specter explaining his party switch outside of 30th Street Station on the day it happened - April 28, 2009 - and the affected, grating way Specter said it was so that he could be "re-e-LECT-ed."

"My idea was, 'OK, play that clip twice in the ad,' " Oxman said yesterday, still basking in the glow of Sestak's once-improbable victory over the five-term incumbent Specter, the longest-serving U.S. senator in Pennsylvania history.

Oxman and the Campaign Group had already crafted some of the most iconic political commercials in Philadelphia history - most notably 2007's so-called "Olivia" ad, which featured Mayor Nutter's then-12-year-old daughter and which is credited with putting him over the top.

But arguably none of the ads by Oxman's firm - including his work for Gov. Rendell or former Mayor W. Wilson Goode - has had the impact, either in terms of a nationally watched race or in boosting an underdog, as the 30-second Sestak ad titled, simply, "The Switch."

The Delaware County congressman and former admiral trailed Specter by roughly 20 points for months. Although he was drawing closer when the ad debuted just 12 short days before the primary, he surged to a narrow lead almost immediately after.

"I think it was the single most devastating ad in Pennsylvania history," said Franklin & Marshall College political scientist and pollster G. Terry Madonna, who's been watching statewide elections for three decades.

The only spot that came close to having the same impact was 1986's famous "guru" ad that helped elect Democrat Robert Casey governor over the GOP's William Scranton by making light of Scranton's flirtations with transcendental meditation.

Madonna said his own polling showed Democrats' citing Specter's party switch - the commercial's core theme - for why they were rallying behind Sestak.

Oxman will not quibble about the impact. He said a local radio reporter told him that she was interviewing Philadelphia voters on election eve and that many took joy in reciting the key line verbatim, included Specter's affected "re-e-LECT-ed."

Oxman, a 60-year-old Southwest Philly native whose firm now advises candidates from coast-to-coast, said yesterday that the anti-Specter ad was 18 years in the making.

Oxman explained that he was determined to build on a 1992 commercial he wrote that almost unseated Specter for previously unknown Democrat Lynn Yeakel. That spot featured Yeakel introducing a clip of the then-GOP senator grilling Anita Hill in the Clarence Thomas Supreme Court confirmation hearings by asking, "Did this make you as angry as it made me?"

Now, Oxman tasked his associate J.J. Balaban - who'd handled Sestak's two congressional races - to find footage of Specter talking about his 2009 party switch that would have the same impact. "If we don't have an ad like this," Oxman told Balaban, referring to the Yeakel commercial, "you're going to be working for minimum wage."

Balaban and the firm's longtime producer, Mark Moscowitz, spent months sifting through footage to get the spot just right, including the 2004 tape of George W. Bush endorsing Specter that was like catnip to still-angry liberals.

Oxman bristles at the complaint from Specter's backers that the senator's line was taken out of context because he had been talking in terms of helping Pennsylvanians with his seniority, and not in terms of himself. "It was crap," Oxman says.

The Campaign Group's co-founder, Doc Sweitzer, worked with Oxman to scribble out the tag line, borrowed from a commercial they had done for an obscure New Jersey candidate: "Arlen Specter switched parties to save one job . . . His, not yours."

There was a reason Sestak waited so long to unleash the ad: Money. It costs $1 million a week to do a statewide ad buy in Pennsylvania, and the challenger had only $4 million to spend. In fact, Sestak filmed just four ads, a remarkably low number for a major candidate in a large-state election.

But in the end, only one mattered.