Sen. Arlen Specter thought he was avoiding the primary from hell when he left the Republican Party three months ago, only to find a stiff challenge awaiting him in the Democratic contest from a former Navy admiral with a golden resume.

U.S. Rep. Joe Sestak, addressing cheering supporters in Folsom, made it official yesterday, vowing to bring "change and accountability" to the Washington insiders who he said had let down the nation.

The announcement was long-anticipated after weeks of Sestak's attacks on Specter, a GOP senator for 28 years, as an unreliable Democrat who was about to be handed the party's nomination by the White House and Gov. Rendell for expediency's sake.

In the past, the stubbornly independent Specter had to tack to the right to quiet the dominant conservatives in the GOP. Now, he has to shore up his left flank - and some measures show Specter voting more often with the Democrats since he switched parties and Sestak began threatening to run.

Specter said yesterday that "it's a matter of experience."

"The overarching issue is who can do more - and what I have done on bringing jobs to Pennsylvania, money for health care, the leadership I've had," he said in an interview.

Sestak launched his announcement tour of the state in a VFW hall crowded with more than 100 supporters chanting "Joe" as four ceiling fans circulated stuffy air and Bruce Springsteen's "No Surrender" blared from speakers.

Sestak, 57, recounted his resume, from his attendance at Cardinal O'Hara High School in Marple Township to his naval commands and his job on former President Bill Clinton's National Security Council.

He said he wanted to improve health care, education, and veterans benefits and to fight for the middle class, though he did not dwell on specifics.

"These problems, when I am your senator, are not going to just disappear overnight," he said. "I can't promise you that. But I can promise you that you will have working for you the hardest-working senator. That you will have the most honest of senators, the most accountable of senators. You will have the most caring of senators. And you will have the senator with the most energy."

Sestak did not mention Specter's name. Rather, he invoked Barack Obama, saying that his own congressional election in 2006, like the president's last year, represented change. Among the poster-size black-and-white photos on the back wall of the VFW was one of Obama shaking Sestak's hand.

"If we do everything by political calculation as a party, where will we get the audacity to really further our ideals?" Sestak told reporters after the event. If Obama, who spoke of the "audacity of hope," had listened to naysayers when he launched his campaign, Sestak said, he would not be president.

Political analysts said Sestak's best hope was to mount an insurgency.

"It's a small one, but there's an opening for Joe Sestak to run a pitchforks-and-torches sort of campaign, storming the gates of the establishment," said Philadelphia-based Democratic strategist Mark Nevins, who is not involved in the race. "His whole profile sets up for that. . . . Anybody else would be toast."

As it is, Sestak faces an uphill battle against Specter, 79, who is backed by the White House and most Democratic leaders nationally and in the state. The challenger has $4.3 million in campaign cash, while Specter has $7.6 million, federal reports show.

Showcasing his connections, Specter hosted Health and Human Services Secretary Kathleen Sebelius in Philadelphia last weekend and is scheduled for several visits to the state with at least two other cabinet members in the next couple of weeks.

"With President Obama behind me, that's a lot of good for Pennsylvania," Specter said.

Sestak, who did a whirlwind tour of the state earlier in the summer, has been encouraged by the party's influential online liberal activists. He said he would appeal to the grass roots.

"When I went around those 67 counties, not one person said to me . . . 'Joe, who do you know?' " Sestak said after his announcement. "This is the ideal situation. I love being an underdog."

The 67-county tour has been a Specter trademark since he was first elected to the Senate in 1980, and it was telling that Sestak aped it.

Specter said: "When I cover the counties, I have town meetings and I talk to people. You can barely drive through the 67 counties in three weeks."

On April 28, Specter defected to the Democrats as his political situation in the GOP was deteriorating. In the wake of his crucial vote for Obama's stimulus plan, polls showed him losing ground to primary challenger Pat Toomey, the conservative former congressman who almost beat him in 2004. Specter said at the time that he could not win the Republican nomination.

Since then, Specter has voted with the Democratic majority in the Senate nearly 93 percent of the time, according to a Washington Post database of congressional votes.

While still a Republican, Specter broke ranks this year to vote with Democrats 44 percent of the time, according to an analysis by Nate Silver, the statistician and blogger who runs the site FiveThirtyEight.com. That climbed to 69 percent over his first few weeks as a Democrat. Since Sestak announced he was leaning toward a challenge, Specter has voted with Democrats 97 percent of the time, Silver found.

Beyond the numbers, Specter sided with the administration and his new party's leaders on some weighty matters, including a controversial bill to cap emissions of greenhouse gases.

"Where will he be in the future?" Sestak asked in an afternoon conference call with reporters.

Specter's campaign gave Sestak a rough introduction to the race, blasting him for missing 105 votes in the House this year, the highest number among Pennsylvania representatives.

Sestak decried the "petty insults and personal recriminations" and fired back.

"I think the 'missed vote' was where Arlen Specter voted for privatizing Social Security," he said. "The 'missed vote' was where he voted for the Iraq War" and for George W. Bush's tax cuts for the wealthy.

"The number-one job a member has is to vote, and he's taken a lot of time off," Specter said. "I think it's a factor."

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