Sen. Arlen Specter yesterday tried to snuff speculation that he was preparing to bolt the Republican Party in the face of a conservative uprising that threatens his bid for a sixth term.

"To eliminate any doubt, I am a Republican, and I am running for reelection in 2010 as a Republican on the Republican ticket," Specter said in a statement released by his campaign manager.

The clarification came after Specter told the Hill newspaper that he would not rule out becoming an independent and caucusing with Senate Republicans - as Sen. Joseph Lieberman (I., Conn.) did in 2006 with the Democrats.

"It's pretty hard to run without a party," Specter told the Hill. "It's always something that could be a possibility. But then I wouldn't be in the Republican caucus - wouldn't have quite the standing as a Republican."

Conservatives have been furious with Specter because he was one of three Republican senators whose votes clinched passage of the $787 billion Democrat-drafted economic stimulus package. He also faces skepticism in the party's base because he has not come out against legislation that would make it easier for labor unions to organize.

Former Rep. Pat Toomey, a conservative Republican from Allentown, has said he is likely to challenge Specter for the party nomination. In 2004, Specter beat Toomey by 2 percentage points. Antiabortion activist Peg Luksik also wants to run.

Pennsylvania law does not allow somebody to run as an independent after losing a primary, as Lieberman did.

In addition, Pennsylvania has little history of successful third-party candidates, said G. Terry Madonna, pollster and professor at Franklin and Marshall College in Lancaster. "We still have very high high partisan identification in our state, which is a huge difference from Connecticut," he said.

Specter told The Inquirer on Tuesday that he believed independents should be allowed to participate in Pennsylvania's primaries, now limited to enrolled members of a party. "We need to broaden citizen participation," he said.

There also had been speculation Specter would become a Democrat, fueled largely by Gov. Rendell's televised comments. Specter has always dismissed that talk.