Rep. Joe Sestak (D., Pa.) has begun telling close supporters that he intends to run against newly Democratic Sen. Arlen Specter in the 2010 primary, setting the stage for a battle that party leaders hoped to avoid.

The disclosure came in handwritten letters Sestak sent to close supporters in recent days, along with a plea for more campaign cash by the June 30 Federal Election Commission reporting deadline.

"My candidacy's credibility will have much to do with my fund-raising success," Sestak wrote in one version, first reported by the Web site Talking Points Memo.

In an interview last night, Sestak said that he needs to consult with his wife, Susan, and their 8-year-old daughter, Alex, before launching what would be an all-consuming statewide campaign.

"Personally, I do intend to get in, but we make the decision as a family. This is a deployment," said Sestak, a retired Navy rear admiral. "We have not made a final decision. . . . I intend to try to do this in a thoughtful, deliberate way."

He added that, in preliminary conversations, his family had been "very supportive" of the idea.

After more than four decades as a Republican, Specter switched parties last month and is running for a sixth term as a Democrat - with the full backing of the party's powers, including President Obama and Gov. Rendell.

Sestak, who had been considering the Senate race before Specter's switch, had more than $3 million in his campaign treasury at the end of March - significant seed money, but short of what most experts believe could be a primary that costs tens of millions.

"Fund-raising has an immediate impact on credibility.. . . I recognize the practical side of politics," Sestak said.

He said his decision would be driven by issues and his family rather than how much cash he could raise in the next month. He said he would announce a decision in the "next number of weeks."

Amid Memorial Day events Monday, Sestak told The Inquirer that "odds are great" he would run and that he was "moving closer to getting in."

Sestak has been highly critical of Democratic leaders for anointing Specter as the presumptive nominee and questions whether the former Republican can be relied on to advance the party's agenda.

"I'm not convinced he's the right person to bring change in our economy, health care, and education through 2016," Sestak said.

His critique has resonated with liberal Democratic activists and union leaders angry at Specter's opposition to a bill that would make it easier to organize workers. The question is how big the base is for an alternative to Specter.

In recent weeks, Specter has been involved in negotiating a compromise on the pro-union legislation, and has softened his earlier opposition to a government-run health plan to compete with private insurers - a central element in Obama's proposed reform of the health-care system.

Specter, 79, was scheduled to attend a Democratic National Committee fund-raiser last night with Obama at the Beverly Hilton in Los Angeles. Academy Award-winner Jennifer Hudson and Earth, Wind and Fire were to provide entertainment for the event, at which party officials expected to raise $3 million to $4 million.

Sestak, 57, was elected in 2006, defeating longtime incumbent Curt Weldon.

Sestak said nobody from the White House had contacted him to urge that he stay out of the race, though he has traded phone messages with Sen. Robert Menendez (D., N.J.), chairman of the Democratic Senatorial Campaign Committee.

"I'm kind of low on the totem pole, and they have a lot of big fish to fry," he said.

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