Sen. Arlen Specter is making his last stand before a new "jury," as he has called it, of Democratic primary voters. Until a year ago, they were the enemy.

With the hours slipping away until the polls open Tuesday, Specter finds himself in a tight race with Rep. Joe Sestak, despite the untold millions of dollars he has brought to Pennsylvania over three decades of service, the support of the White House, and backing from almost every union and Democratic organization.

Specter, it turns out, is haunted by his Republican past. Many Democrats, habituated to voting against him, are skeptical of a man they hold responsible for helping put conservative Justices Clarence Thomas, John G. Roberts Jr., and Samuel A. Alito Jr. on the Supreme Court, and for supporting the economic policies of former President George W. Bush.

"It's hard to convince the Democrats who vote in closed primaries in midterm-election years that he is one of them," said Lara M. Brown, a political science professor at Villanova University. "These are loyal, year-in-year-out Democrats, and many feel they can't get a fix on where he stands."

Analysts say Specter also faces something even bigger than managing a party change: a toxic environment for incumbents of both parties. Sen. Robert Bennett (R., Utah) was toppled last week; Rep. Alan Mollohan (D., W. Va.), renowned for bringing home federal money, also fell; and Sen. Blanche Lincoln (D., Ark.) is imperiled by a primary challenger from her left.

In short, the election has not really been about Sestak, a former Navy admiral, or even the issues; the stated positions of the two men are almost indistinguishable, standard-issue Democrat. It is and always has been, analysts say, a referendum on Specter.

And he has shifted his positions. During 28 years as a Senate Republican, Specter voted 58 percent of the time with the Republican position, according to Congressional Quarterly. That rose to 70 percent in the final two years of Bush's term. Since converting, he has backed the Democratic position a little more than 95 percent of the time.

Still, Specter enjoyed a double-digit lead in the polls until recently, though beneath the head-to-head matchup were contradictory data. Most voters told pollsters it was time for a new senator, but Democrats gave him approval ratings in the 70s.

Then came The Ad.

Made by Philadelphia's the Campaign Group, the 30-second spot shows Specter, as a Republican, endorsed by then-President Bush as a "firm ally," then arm in arm with Bush and then-Sen. Rick Santorum (R., Pa.). Footage follows of Specter saying, with a satisfied grin, "My change in party will enable me to be reelected." The tagline: Specter switched to "save one job . . . his . . . not yours."

When he switched in April 2009, after his decisive vote for President Obama's $787 billion stimulus plan had destroyed his standing in the GOP, Specter said he could not win reelection in a Republican primary. He also said that he wanted to continue to do good things for the state and nation, and that the GOP had moved too far to the right for moderates like himself.

But it is the bluntly honest remark about reelection that has stuck, especially after Sestak reinforced it with $1.6 million in advertising.

"From the time he savaged Anita Hill, I decided I'd never vote for him," said Democrat Patricia Bannon, 85, of Westtown, recalling Specter's tough 1991 questioning of the woman who challenged Thomas' court nomination. "He was a Republican then, so it didn't matter. Now he's a Democrat and he can wave around Obama or whoever he wants, I'm not voting for him."

Specter would have been better off had he finessed his party switch, said Democratic strategist Mark Nevins.

"He never took the time to convince voters that it was a principled decision based on values, shared beliefs, and common goals," Nevins said. "He was very up-front about the personal benefit. There was enough cynicism to it to make Democratic voters question whether this guy really shared their values."

Specter has countered Sestak's "switch" ad with one of his own, featuring Obama crediting him with "helping pull us back from the brink" with the stimulus vote. "I love Arlen Specter," Obama says.

By the time the polls close, that image will have been seen by the average broadcast TV viewer in the Philadelphia market 12 times - and more in Pittsburgh, Scranton, and Erie - according to summaries of the campaigns' ad buys. Sestak's latest ad will be close to that number in all those markets but Erie.

The campaign became a slugfest about character. In Sestak's telling, Specter has no principles. In Specter's telling, Sestak lost a top Navy post because he treated subordinates poorly, has missed 127 votes in the House, and pays his campaign workers below the minimum wage.

While portraying himself as taking the high road, Sestak also stoked the implicit issue of Specter's age and health; the senator, 80, has survived two bouts of Hodgkin's lymphoma and a brain tumor. Sestak, 58, rarely fails to mention his admiration of Specter's "long service" and says it's time for "new blood." In his last two TV ads, Sestak has used mug shots of a bald and haggard Specter, taken when he was recovering from chemotherapy in 2008.

In his closing ad, which boasts of Sestak's higher ratings by various Democratic interest groups, the campaign also presses the message that Specter has been around too long. A narrator calls the congressman "the best Democrat for Pennsylvania's future." Sestak himself says, "It's time for a new generation of leadership."

To Specter, the use of the post-chemo pictures was low.

"There's a great deal of latitude in politics, but it's way below the belt to publish a picture of a guy when he was bald, when all his hair has since grown back," Specter said Friday in an interview. "Way below the belt, but typical of my opponent."

If he is to survive, Specter will need a big vote from Philadelphia, his place of residence and his strongest area, and other urban centers. His campaign combined forces with the state AFL-CIO and state and national Democratic groups to knock on 20,000 doors last weekend, and will hit 20,000 more by the end of this weekend, campaign manager Chris Nicholas said.

Sestak has built networks of volunteers across the state, but he has spent the bulk of his cash, a little more than $4 million so far, on TV and radio ads.

One thing in Specter's favor is the intangible: He's been here before. Specter has a habit of winning close races in which his political obituary has been pre-written. He beat Democrat Lynn Yeakel in 1992, the "Year of the Woman," by 3 percentage points. In 2004, Specter beat Rep. Pat Toomey, 51 percent to 49 percent, in the GOP primary.

This time, it will likely come down to whether Specter has made enough Democrats comfortable with him as a Democrat.

Specter has won over Vincent Johnson, 45, an investigator for the City Controller's Office, who was lunching Friday on potato soup in JFK Plaza, where Sestak was campaigning. Johnson cited Specter's support for the stimulus, the health-care overhaul, and Obama's agenda.

"I'm willing to forgive him for having been a Republican as long as he's willing to work for the American people, at the side of the president," Johnson said. "And he is."

Contact staff writer Thomas Fitzgerald at 215-854-2718 or tfitzgerald@phillynews.com.