After weeks of hints and almost-declarations, U.S. Rep. Joe Sestak plans to make it official today: He will challenge veteran Sen. Arlen Specter in the 2010 Pennsylvania Democratic primary.

The second-term congressman from Delaware County, a former Navy vice admiral, scheduled a morning rally at his home VFW post in Folsom, followed by a two-day tour that will take him to Pittsburgh, Johnstown, Harrisburg, and Scranton - ending with an appearance tomorrow on Comedy Central's The Colbert Report.

Specter, after four decades as a Republican, nearly three of them in the Senate, switched parties in April, saying the GOP had become too conservative for him to win election to a sixth term.

While President Obama, Gov. Rendell, and much of the party leadership immediately embraced Specter, Sestak is betting that many rank-and-file Democrats won't trust the change. As he barnstormed this summer across the state, Sestak attacked what he called an effort by Washington elites to "anoint" Specter the party nominee.

"It's kind of David vs. Goliath - an insurgency versus the establishment," said pollster G. Terry Madonna of Franklin and Marshall College. "The irony is Sestak is equating it with Obama's battle against Washington and the way it does business."

But Specter, 79, is a household name, while Sestak, 57, is little known beyond his congressional district in suburban Delaware, Chester, and Montgomery Counties. Specter also has long shown an independent streak and an ability to appeal to Democrats.

The support of a popular president is also an asset, and White House officials said yesterday that Obama planned to join Specter for a Philadelphia-area fund-raiser in mid-September in support of his reelection.

Recent polls, though early, have shown a softening in support for Specter, though he still leads Sestak in head-to-head matchups.

A Quinnipiac University survey released July 22 found voters, by a margin of 49 percent to 40 percent, saying Specter does not deserve reelection - a reversal from the university's May poll. The telephone poll of 1,173 registered voters had an error margin of 3 percentage points.

In late June, a Franklin and Marshall poll of 580 respondents found that 28 percent said Specter deserved reelection, down from 40 percent in the college's March survey. The error margin was 4.2 points.

"Democrats might respect Specter, they might not dislike him, but I don't know if they love him," said Christopher Borick, a political scientist and professor at Muhlenberg College. "The name of the game in primaries is, can you bring out people who feel passionately about you."

At the same time, Borick said, Sestak has to "give Democratic voters something they can be energized about" beyond stoking dissatisfaction with Specter. And that will take lots of money.

The most recent campaign-finance reports showed that Sestak had $4.3 million in cash on hand as of June 30, after raising $1 million since March. But Specter had $7.5 million.

"Basically, $4 million gets Sestak an invitation to the party, but how is he going to stay competitive or win a campaign that's going to cost from $10 [million] to $15 million?" Democratic consultant Larry Ceisler said. "For the most part, he's going to be cut off from traditional sources of fund-raising, because Democrats from Obama on down are supporting Specter."

It's far from clear that Sestak can count on the fund-raising help he received from President Bill Clinton's network in his first congressional race, in 2006, Ceisler said. If Sestak pursues the Senate race, he will vacate his House seat, sparking a hotly contested race for the Seventh Congressional District.

Skirmishing between Sestak and Specter has broken out several times.

After Sestak blasted the party-swapping senator as "not a real Democrat," Specter pointed out that Sestak himself had not registered as a Democrat until 2006, just before he ran for Congress. Specter called him a "flagrant hypocrite."

Sestak responded that he had been registered as an independent because he did not believe military officers should be partisan.

Contact staff writer Thomas Fitzgerald

at 215-854-2718 or tfitzgerald@phillynews.com.