A federal judge yesterday upheld the Abscam bribery conviction of Sen. Harrison A. Williams Jr. (D-N.J.), rejecting the senator's claims that government misconduct violated his constitutional rights.

The 61-page ruling by U.S. District Court Judge George C. Pratt was expected by most trial observers. Williams' attorney said appeals will continue, but the decision is the last legal step before the Senate considers Williams' expulsion from Congress next month.

Pratt, who set sentencing for Jan. 26, said: "On the entire record it is clear that defendants entered on a course of conduct knowingly and voluntarily, and that before committing the criminal acts charged against them they were aware that corrupt use of Sen. Williams' political influence and power was to be an essential part" of the deal.

A jury in Brooklyn convicted Williams and co-defendant Alexander Feinberg on bribery and conspiracy charges May 1. It found the senator had agreed to trade his influence for a hidden share in a titanium mine to be financed by a $100 million loan from an undercover FBI agent posing as an Arab sheik.

Williams was the seventh member of Congress, and the only senator, convicted of corruption charges stemming from the FBI's controversial Abscam "sting" investigation.

His case was considered the most difficult for the government because of the complexity of the titanium deal, his refusal of a direct cash bribe offer, and undercover informant Melvin Weinberg's coaching of Williams on what he should say at a key meeting with the "sheik" in June, 1979.

Williams' attorney, George Koelzer, argued at trial and in his due process motions that the FBI agents' conduct was outrageous and that they had entrapped the senator. But Pratt said, "There is no way that Williams could have misunderstood what was proposed: an offer of Williams' 'power and influence' to obtain contracts in order to induce the sheik to grant the loan."

Pratt said Williams "acted voluntarily and intentionally in that meeting with the sheik and was not influenced to say or do anything that he had not previously agreed to say and do; consequently Williams was not prejudiced by the conduct of Weinberg . . . . "

In a statement last night, Williams said: "Judge Pratt's decision comes as no surprise . . . we welcome today's decision so we can take the case . . . into the United States Court of Appeals, where we are certain that Judge Pratt will be reversed . . . . "

Thomas P. Puccio, the chief federal Abscam prosecutor, said he was pleased with Pratt's ruling and recommended it as "required reading for all those who wish to know what the true facts and issues in this important case really are."