U.S. Rep. Joe Sestak says that he isn't holding out for a deal. He's arming up for the fight of his life. The former three-star Navy admiral, who took down Rep. Curt Weldon in 2006, is crisscrossing Pennsylvania in the dead of night as he prepares to challenge five-term Sen. Arlen Specter, the consummate political survivor, in the Democratic primary.

Some critics have called it a kamikaze mission, an act of hubris by a second-term Democrat who doesn't know his place. Others question whether he'll make good on the threat, or if he'll pull out when the right offer comes along.

Sestak isn't hearing it.

"What's moved me into this is I didn't like the deal that was made," he said of Specter's defection to the Democratic Party, and the way its leaders immediately embraced him as their candidate. "I don't believe in deals. Do you think I'm going to take a deal to get out?"

It's a bold statement from a guy who can afford to make them. Having transitioned directly to Congress after 31 years in the Navy, Sestak, 57, doesn't owe many favors to politicians and power brokers, which makes it nearly impossible to rein him in, even in the face of such a daunting challenge. His closest advisers are relatives.

"Joe is like a ballplayer who is loved by the crowd," said Cliff Wilson, chairman of the Delaware County Democratic Party. "When he gets out on the field the crowd goes crazy, but back in the dugout he's not the most popular guy with the teammates."

President Obama, for one, is backing Specter. So is Vice President Biden, who helped lure Specter away from the GOP in what Washington Democrats see as a major coup - and a potential filibuster-breaking 60th vote in the Senate. Pennsylvania's junior senator, Bob Casey, is standing by his colleague.

Gov. Rendell thinks that Sestak would "get clobbered" in next year's Democratic primary, ticking off the reasons why Sestak has "practically no chance" against Specter, who is well-known across the state and likely will have an inexhaustible supply of campaign cash.

"What in God's name is he doing?" Rendell exclaimed.

Barring unforeseen circumstances, Sestak is getting into the race, and it appears that nobody can talk him out of it - not even the president himself, nor Obama's chief of staff, Rahm Emanuel, a master arm-twister.

"I hope in the general election to have their endorsement," Sestak said in a lengthy interview this week at his office in Media.

So, why risk a promising political career to run against Specter, now a Democratic incumbent?

Sestak said he simply doesn't trust Specter, the longest-serving senator in Pennsylvania history, to support Obama's agenda on health care, the economy and education. And he's finding that others - in Congress, on the Internet and across the state - feel the same way.

"I'm concerned about the flight risk of Arlen," he said of Specter, 79, a longtime Republican until he switched parties in April. "After this election, will he be there consistently on the important issues in the way that Pennsylvanians need him to be?"

And, as Sestak has said probably hundreds of times by now, Specter's "anointment" as the Democratic candidate really raised his hackles.

"This isn't right," he said.

Democratic sources told the Daily News that, prior to Specter's party switch, Sestak was being courted by the Democratic Senatorial Campaign Committee to challenge the Republican nominee - most likely Specter or Pat Toomey - in the 2010 general election. Now, the committee is seeking to clear the field for its newest member, to avoid a contested primary.

"As a general rule, we like to avoid primaries," said DSCC spokesman Eric Schultz.

Schultz declined to comment on whether the DSCC had attempted to recruit Sestak as a candidate, as did Sestak, who abhors inside-the-Beltway politics.

Early polls give Specter a massive lead over Sestak, who has little name recognition outside his Delaware County-based district.

"It's not [just] that nobody knows him west of Harrisburg," Rendell said. "It's that nobody knows him in Bucks County, nobody knows him in Philadelphia."

But Specter's support may be soft. A recent labor-funded poll of likely Democratic voters, conducted by Greenberg Quinlan Rosner Research, shows that Specter leads Sestak by only 8 points when respondents are read a positive profile of each candidate. Among voters who already know both candidates, Sestak actually leads Specter 52-44.

Raising the kind of money Sestak needs to get his message out will be difficult, with Democratic leaders leaning on donors to contribute to Specter. The congressman has a respectable $3.5 million in the bank, but Specter already had about $6.7 million at the close of the last quarter.

"If he does run against Arlen," said Upper Darby GOP Chairman John McNichol, "[Sestak] doesn't have a prayer."

But he has a strategy. It consists not solely of attacking Specter from the left, but flanking him on both sides by gathering support from liberal Democrats as well as moderate Republicans, including those who joined the party last year to vote for Obama or Hillary Clinton.

"There are some solid advantages for Specter - given his name recognition, the cast of supporters he has and his financial resources - that make him the front-runner," said Christopher Borick, director of Muhlenberg College's Institute of Public Opinion.

"But I truly believe there are soft sides to his front-runner status. For Democratic voters that are seeking to find a candidate that matches them as well as possible, Specter absolutely has liabilities."

Sestak, who said that 25 of the state's 67 county Democratic organizations have expressed interest in supporting his candidacy, will be looking to veterans groups and "netroots" activists for money.

This week, Daily Kos founder Markos Moulitsas and OpenLeft's Chris Bowers, who is also a Democratic state committeeman, voiced their support for Sestak, who raised a large sum of money online during his 2006 campaign.

Also, 63 percent of Democrats believe that Specter should earn their party's nomination in a primary, according to a recent poll by Susquehanna Polling and Research, a Republican firm.

Specter got a warm welcome last weekend at the state Democratic Party's committee meeting in Pittsburgh, where he received five standing ovations. He cast off his Republican credentials, referring to the GOP as "obstructionist," and said, "I'm again a Democrat, and I'm pleased and proud to be a Democrat."

But at a labor rally there, he got a mixed reception from union members who are anxious to see how he votes on the Employee Free Choice Act, which would make it easier for unions to organize. Specter assured them that he's working on a compromise and said, "I believe you'll be satisfied with my vote on this issue." Sestak is a co-sponsor of the act.

Sestak clearly is not going to win the battle for big-name endorsements, but some elected officials already are raising concerns about nominating Specter.

Montgomery County Commissioner Joe Hoeffel, a former congressman who ran against Specter in 2004, said that he would back Sestak in the primary. He has cited Specter's support for President Bush, whom he called "the worst President this country has ever had."

U.S. Rep. Barney Frank, D-Mass., chairman of the House Financial Services Committee, said this week that he's concerned about Specter's "erratic behavior," and questioned whether he's a true Democrat.

"I don't think he's as reliable a vote for the things I care about as another Democrat would be," Frank said, adding that he'd back Sestak over Specter.

Specter's campaign manager, Christopher Nicholas, said yesterday that the senator considers health-care reform a "top priority" and pledged to work with Obama if re-elected. But Specter knows that Obama's endorsement doesn't mean a clear path to re-election.

"The senator's always had difficult primary campaigns," Nicholas said. "This one, if it pans out, will be no exception. We're ready to fight it out."

Despite some Republicans' doubts that he'll run, Sestak doesn't appear to be bluffing. After the interview Sunday night, he awoke at 1:30 a.m. to drive five hours to Pittsburgh to meet with reporters, then drove to Washington Monday night.

He can function on minimal sleep, a trait that he attributes to his years commanding ships at sea.

"If people are investing their time and resources in me, I want them to know I'm going all-out," he said of his round-the-clock campaign style.

David Landau, an adviser to Sestak in 2006, said that Sestak has to "run the perfect campaign" to earn the Democratic nomination. He said that the race could hinge on how heavily Obama gets involved, but warned against dismissing Sestak, whose campaigns resemble a "military operation."

"Arlen Specter may work hard, but it pales in comparison to Joe Sestak," Landau said. "You should never underestimate him. If anybody could come into this situation, given the odds against him, and beat Arlen Specter, it'd be Joe Sestak."