READING — Arlen Specter twisted around from the podium to peer at the men standing behind him at a recent endorsement event, as though to remind himself who exactly they were.

All were Democrats and Specter, Pennsylvania's longest-serving U.S. senator, had been accustomed to endorsements by Republicans — until he switched parties a year ago after spending more than 28 years as a Republican senator.

Specter, a centrist who has pressed for biomedical research funding, abortion rights and hate crime penalties, brushed aside questions about the difference between this campaign — his first as a Democrat — and those that came before.

"My voting record — pro-labor, a woman's right to choose, civil rights — stays the same," Specter, 80, said after the event at Reading's Goggleworks art center.

Specter's run for a sixth term is almost sure to hinge, once again, on his record: Voters on the political right, on the left and in the middle are holding it up to the light to try to decide whether a man who prides himself on his independent streak — and his ability to infuriate either side of the political spectrum — is acceptable enough to support.

Specter's principal opponent in the May 18 Democratic primary is Joe Sestak, a second-term congressman and retired Navy vice admiral from suburban Philadelphia who raised eyebrows with the claim that the White House offered him a job to get him to drop his challenge.

Also in the race is political newcomer Joseph Vodvarka, who is retired from his spring manufacturing business near Pittsburgh.

The Republican primary pits former Lehigh Valley congressman Pat Toomey against Johnstown activist Peg Luksik. Toomey won the Republican party endorsement and has a strong name among conservatives — he came within about 17,000 votes of knocking off the party-endorsed Specter in the 2004 Republican primary out of 1 million cast.

Polls show Toomey with a lopsided lead over Luksik in the GOP primary. In hypothetical general-election matchups, Toomey has led Sestak while he and Specter have traded leads in different polls.

In the Democratic primary, Specter has led Sestak in recent polls by more than 2-1. As of Dec. 31, he also had more cash on hand — $8.7 million to Sestak's $5.1 million.

Sestak, 58, contends the gap will close when he begins airing television ads and voters get to know more about him.

The highest-ranking member of the military ever elected to Congress, Sestak attacks Specter as lacking conviction in Democratic principles and owing his political survival to dealmaking with power brokers.

"I would have run against Arlen Specter as a Democrat or a Republican," Sestak said in a recent interview.

"He's just voted the wrong way. His generation has had its time — it's past — where politics has prevailed too often. Even changing your party over principle."

Specter's record includes seeming contradictions that are difficult to explain.

In 2004, for example, he was endorsed by conservative U.S. Sen. Rick Santorum and President George W. Bush over Toomey while describing himself as a "true Republican." In this campaign, Specter is being endorsed by the Democrat who beat Santorum in 2006, Bob Casey, and President Barack Obama, Bush's Democratic successor.

Specter voted in 2001 for Bush's $1.35 trillion tax cut package when three out of four Democratic senators opposed it as a giveaway to the wealthy. Specter also voted for the GOP's John McCain-Sarah Palin presidential ticket.

He defends his record as in line with Democrats more often on core issues, such as the minimum wage increase.

"You can pick at 10,000 votes, you can find a lot of votes to support most any proposition," Specter said. "But that record has won me a lot of support among Democrats."

He's not without credibility among Democrats.

In 2004, the AFL-CIO of Pennsylvania endorsed Specter over the Democratic challenger, then-U.S. Rep. Joe Hoeffel, after he fought for tariffs on foreign-made steel.

In February, the state Democratic Party endorsed Specter over Sestak. And the Berks County Democrats who endorsed him gushed over his frequent visits and tireless work to deliver project money to the area as though he had always been a Democrat.

In the past 18 months, Specter has voted with Democratic leaders on major issues, including the Wall Street bailout, stimulus package, greenhouse gas reduction, Supreme Court nominee Sonia Sotomayor, jobs and health care overhaul.

Sestak supported those efforts, too, but says Specter continues to defy Democrats' and Obama's agenda.

As recently as January 2009, Specter supported an unsuccessful GOP amendment to narrow a Democratic bill ultimately passed by Congress last year to make it easier for women, minorities and others to sue for pay discrimination. Still, Specter supported the final bill, while seven out of eight Republican senators did not.

For Specter, the beginning of the end of his rocky membership in the Republican party began in February last year when he supported Obama's stimulus bill.

It was a risk that no other Republican in Congress facing re-election in 2010 was willing to take and Specter soon concluded that he couldn't persuade the state's 3.1 million Republicans to support him one more time.

So he set about wooing the state's 4.3 million registered Democrats.