U.S. Rep. Joe Sestak has been a fundraising juggernaut during his first re-election bid, collecting more than $3.6 million in campaign contributions, but it's the relatively modest amount that he's spent which is raising eyebrows.

The Edgmont Democrat has raised more money this election cycle than all but one of his fellow Pennsylvania Democratic incumbents (fellow freshman U.S. Rep. Patrick Murphy edges him by barely $90,000).

Two weeks before Election Day, however, Sestak had spent only about $880,000 in his race against Republican Craig Williams in the 7th Congressional District, according to campaign finance data. It's no paltry sum: Sestak has spent on direct-mail, some newspaper ads and building a sprawling ground-game and get-out-the-vote operation. But in the context of Congressional contests, some would call it a pittance. More telling, Sestak, as of Friday, had spent nothing on the television or radio advertisements that are a staple of most Congressional campaigns, his campaign spokesman confirmed. The decision has left the campaign with more than $3 million in the bank.

Murphy, by contrast, has spent more than $2.3 million on his race against Republican Tom Manion, showering the airwaves with TV ads. While his race was initially expected to be more competitive, it has dropped off many analysts' radar screens in recent weeks, as the economy has overtaken the Iraq war as the primary issue. It is now considered only slightly more competitive than Sestak's race, and a couple political watchers even consider it less competitive.

So why is Sestak, a former three-star admiral, spending so lightly?

Even though Sestak has voiced concern - in emails to supporters - that the race will be a close one in a district where Republicans still outnumber Democrats by about 50,000 voters, few political analysts expect Williams to be serious competition for Sestak.

Williams, a former federal prosecutor and marine who is widely considered to have a bright future in the GOP, admittedly has a name recognition problem, and the race has been very low on political forecast lists of races to watch.

So perhaps it's not surprising that Sestak is largely sitting on his overflowing campaign war chest, even as his opponent floods the airwaves and newspapers with as much advertising as his more meager fundraising will allow.

"I would assume he feels very comfortable with his chances or he wouldn't be sitting on the wad of cash that he is," Chris Borick, a pollster and political science professor at Muhlenberg College, told PolitickerPA.com.

Conventional wisdom would call for a freshman incumbent with Sestak's money to spend heavily and pile up huge margins of victory to scare away future challengers. That he's not doing so has spawned all kinds of speculation among political insiders.

The favorite theory: Sestak is saving his cash for what would be an expensive 2010 U.S. Senate race.

It wouldn't be far-fetched. With his fundraising prowess and his ability to appeal to moderate Republicans and independents, Sestak is already one of the names being floated as a possible Democratic candidate to take on Republican U.S. Sen. Arlen Specter in two years.

U.S. Rep. Allyson Schwartz (D-Abington) and MSNBC commentator Chris Matthews have also been frequently discussed.

In an interview Friday, Sestak campaign spokesman Chris Ortman would not indulge in any speculation, saying his boss was solely focused on winning re-election.

"He loves the job of being a Congressman, and he's going to keep doing the job because there's still plenty of work to be done," he said.

Ortman said the campaign would be spending more money on advertising in the closing days before the election, though he wouldn't go into details about what kind of ad buys were in the works. He rejected the premise that the campaign hadn't been shelling out cash for the race.

"The perception that we're not spending money is incorrect," he said.

And, he noted, Sestak was visiting all 67 municipalities in the district before Election Day, while building what the campaign is calling a "volunteer army" for turning out voters.

"We've invested in building a strong grass-roots base of support," Ortman said.

There are other possibilities as to why Sestak would be holding onto so much cash. This fall is about as friendly an electoral climate as Democrats have seen in decades. If the party controls both the White House and Congress for the next two years, 2010 could prove to be a tougher election, especially without a presidential race to bolster turnout.

"I think he's probably saving it for his 2010 reelection to Congress," said Anthony Campisi, a vice-chair of the Delaware County Democratic Party. "We're in a favorable political environment. Raise the money you can is their approach, I guess. Who knows what the political environment will be in 2010. Things change very quickly."

Sestak could also be saving the money to help out fellow Democrats, in the hopes of moving up the party leadership ladder.

But after Election Day, all eyes in Pennsylvania will be on the big 2010 races for Senate and governor, and Sestak is sure to be closely watched for signs he might jump into a Democratic primary to take on Specter.

"Everybody's thinking about that race," Borick said. "I would imagine that anyone who could enter with a nice war chest would advance their possibilities in a wide-open field."

One thing is certain: On the off-chance that Sestak loses, the money will be of little solace.

"It'd go down in the record books as one of the silliest moves ever," Borick said. "I'd be talking about it in my campaigns class if he sat on a war chest and lost."