U.S. Rep. Joe Sestak's staff said yesterday that the Delaware County Democrat was running just for the U.S. Senate, seeking to quell speculation in state political circles that he planned instead to run for reelection to the House.

Talk buzzed among county Democrats and state party leaders after Sestak's staff received House nominating petitions from the Pennsylvania Department of State for the May 18 primary.

"The congressman is running for the United States Senate and will be filing petitions only for the Senate," spokesman Jonathon Dworkin said.

He said the campaign had received a packet of House petitions but wouldn't use them. "Every incumbent member of Congress automatically receives the House petitions, but we asked for petitions for the Senate, and those are the ones we will be circulating," Dworkin said.

Party leaders have urged Sestak to abandon a potentially costly challenge to Sen. Arlen Specter (D., Pa.) and keep his Seventh District seat in Democratic hands.

Last weekend, Sestak failed to win enough votes on the Democratic State Committee to block an official endorsement of Specter, a five-term senator who switched to the party last spring and is supported by the White House and Gov. Rendell.

Sestak wound up with just under 24 percent of the 306 votes cast; he needed 33 percent plus one to stop Specter from getting the prize.

"I don't think the state committee endorsement reflects the thinking of Democratic primary voters," said Anthony Campisi, vice chairman of the Delaware County Democrats. "Saturday's vote is all about returning favors and knowing people for 30 years."

Legally, Sestak could circulate two sets of petitions, and experts say there is no prohibition on running for two offices simultaneously. The U.S. Constitution, however, prohibits a member of either chamber of Congress from holding another position.

The politics of such a move is another matter.

A central theme of Sestak's campaign is that Specter switched parties because of opportunism rather than principle, so hedging his own bets would open him to allegations of hypocrisy. Democratic leaders said Sestak was convinced that he could win the Senate primary in a year marked by voter frustration with politics as usual.

Sestak continues to trail Specter in head-to-head matchups in polls. The incumbent's favorability ratings are low among voters overall, but Specter's approval percentages among Democrats in several polls were in the 60s and 70s. Sestak has $5 million on hand, Specter just under $9 million.

State Rep. Bryan Lentz is running for the Democratic nomination for Sestak's House seat. Lentz stopped his 2006 campaign for the seat at the request of party leaders who thought Sestak, a former Navy admiral with good fund-raising numbers, would make a stronger candidate.

The GOP's likely nominee in the Seventh District is former U.S. Attorney Pat Meehan.

Candidates can begin circulating nominating petitions Tuesday and have until March 9 to get the required signatures. It takes 1,000 to qualify for a spot on the ballot for the U.S. House, 2,000 for the Senate.

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