WASHINGTON - Sen. Arlen Specter (R., Pa.) said yesterday that he did not seek and was not given any assurance that he would not face a primary challenge when he made the decision to leave the Republican Party to rejoin the Democrats.

"I didn't ask that the field be cleared. There was no discussion of that," Specter said on Fox News Sunday when asked about a possible Democratic primary challenge by Rep. Joe Sestak (D., Pa.).

"Everybody ought to run if he or she wants to run," Specter said. "And I'm ready to take on all comers."

Sestak, 57, a former Navy vice admiral, has said he is seriously considering taking on Specter, 79, who is seeking a sixth term. Asked whether he could beat Sestak in a primary battle, Specter said "in a political campaign there's no such thing as certainty."

Specter had been facing a tough Republican primary challenge from former Rep. Pat Toomey, who narrowly lost to him in 2004. The senator stunned Washington with an April 28 announcement that he was switching parties to run as a Democrat. The moderate's defection has made Democrats one vote shy of the 60 needed to overcome filibusters.

Specter was a Democrat until 1965, when he ran successfully on the Republican ticket for district attorney in Philadelphia.