Sen. Harrison A. Williams Jr. (D-N.J.) has taken the offensive against any Senate move to oust him because of his Abscam bribery conviction, telling colleagues they, too, are jeopardized by overzealous prosecutors.

In an impassioned letter Monday, Williams appealed to other senators not to "render precipitous judgment" without considering the larger constitutional and separation-of-powers questions raised by the government's conduct of the undercover investigation that led to his May 1 conviction in Brooklyn federal court.

Williams and a co-defendant were found guilty of bribery and conspiracy for agreeing to trade the senator's government-contracting influence in return for a $100 million loan from an FBI agent posing as an Arab sheik.

"In defending myself, I have always felt that I also am defending the senate as an institution from questionable infringement by the Executive," he said in the letter, which was made available to The Washington Post. "When one member is subjected to the whims of a prosecution determined to make a case by any means necessary, then all members are equally jeopardized."

He added: "A hasty decision to expel me from membership in the Senate or to pressure me to resign would foreclose an opportunity for the full Senate to register an objection to this kind of prosecution."

In a luncheon with Washington Post editors and reporters yesterday, Attorney General William French Smith said that while he wasn't in the government when the Abscam cases were brought, he understood they were "very carefully monitored all the way through" to assure that individual rights weren't violated.

The Senate Ethics Committee voted last week to file formal charges against williams, saying his trial provided "sufficient credible evidence" that he had broken Senate rules. A committee aide said yesterday that a special counsel to present the case hasn't been picked yet.