In his last-ditch effort to stave off expulsion from the Senate next week, Sen. Harrison A. Williams Jr. (D-N.J.) has found an unlikely ally in right-wing ideologue and former presidential candidate Lyndon LaRouche.

LaRouche, a one-time Marxist who spent much of the 1970s breaking up meetings and harassing public figures, has produced a half-hour film in which Williams sets forth his side of the Abscam affair.

A spokesman for LaRouche's political organization, the National Democratic Policy Committee, said the film has been shown to a dozen senators and to labor and political leaders around the country. The group also is conducting a petition campaign on Williams' behalf.

Williams, who said the NDPC represents his most active grass-roots support, described LaRouche and his supporters as "very thoughtful people" and said their research into the constitutional questions raised by the Abscam case have been "very accurate."

He added in a telephone interview yesterday that he had met with LaRouche only once and could not speak to his positions on other matters because "I haven't researched them."

The senator's administrative assistant, Walter Ramsay, emphasized yesterday that the NDPC was acting "totally on its own" in taking up Williams' cause. He described as "routine" Williams' decision to sit for a videotaped interview late last month with NDPC chairman Warren Hamerman.

"Since we are both interested in constitutional rights, it ill behooves us to dictate what a group can and cannot do," Ramsay said.

In literature discussing what he describes as the "frame-up" of Williams, LaRouche offers an analysis of Williams' predicament that hardly seems likely to endear the embattled senator to his colleagues. He describes Abscam as a "treasonous" action designed to break the will of Congress, then goes on to doubt the "moral capacity" of congressman to stand up to it.

"There are members of Congress on both sides of the aisle so degenerate or so swayed by political opportunism that they variously promote or condone policies more hideous than those perpetuated by Albert Speer and others under Adolf Hilter," he writes.

In another pamphlet, LaRouche suggests that the Heritage Foundation, a conservative think-tank, was behind the plot to descredit Williams so it could place its hand-picked successor--Jeffrey Bell of New Jersey, a former speechwriter for Ronald Reagan--in the Senate. LaRouche describes the Heritage Foundation as the "sham opponent of the Trilateral Commission."

LaRouche, 58, owner of a computer cooperative in New York, bounded onto the political scene in 1968 when he founded the National Caucus of Labor Committees out of the wreckage of various radical groups in the 1960s. His organization was dedicated to the proposition that every other group on the far left was a tool of the Rockefeller family, the Ford Foundation and/or the CIA, and it spent more than five years disrupting meetings of organizations ranging from the Communist Party to the United Auto Workers.

In the late 1970s, LaRouche and his followers made a short trip around the back side of the ideological circle from far left to far right, but their confrontational tactics never changed. His campaign for president last year was rife with wild accusations on the order of: "Gov. Hugh Gallen caught in assassination plot," a headline of a campaign press release during the New Hampshire primary.

Meanwhile, Williams has filed suit in federal court attempting to block floor debate on a resolution to expel him from the Senate unless his new defense lawyers are permitted to speak on his behalf during the proceedings. The debate is scheduled to begin Dec. 3.

Robert J. Flynn Jr., one of Williams' two attorneys, claimed yesterday that "at least one" senator had volunteered to represent Williams on the Senate floor. Neither he nor the senator's aides would identify the volunteer.

But Flynn said a tactical decision has been made not to have another senator lead the defense because it would take months of preparation.

For weeks, the Senate Ethics Committee has been showing senators videotapes of Williams' meeting with FBI agents posing as representatives of an Arab sheik, which were presented during his original trial, in an attempt to familiarize them with the case. More than 70 senators have viewed these tapes and been briefed on the evidence against Williams.

Williams' attorneys have been allowed to sit in on these sessions, but not to speak during the presentations. Williams has, however, mounted a low-key offensive among fellow senators by sending out a packet of materials outlining his side of the case.

Among the items in the packet is an affidavit signed by Salvatore Ottavino, a juror in Williams' trial, in which he says he would have voted for acquittal if a Nov. 27, 1979, prosecution memo had been introduced as evidence. The memo said prosecutors felt it necessary to "recontact" Williams "in an attempt to obtain an overt action on his part" that could lead to a conviction.