A federal prosecutor told jurors yesterday that Teamsters union President Roy Williams and insurance broker Allen Dorfman had attempted to bribe a U.S. senator and loot a union pension fund.

Assistant U.S. Attorney Douglas Roller, head of the Organized Crime Strike Force, delivered his opening statement before U.S. District Judge Prentice Marshall and a jury in the bribery-conspiracy trial. The defendants are Williams, Dorfman, a reputed mobster, Joseph (Joey the Clown) Lombardo, a former Teamsters pension fund trustee, Thomas O'Malley, and an employe of the fund, Andrew (Amos) Massa.

"This case is about these five defendants trying to bribe Sen. Howard W. Cannon of Las Vegas, Nev.," Roller said. "The evidence will show that on Jan. 10, 1979, Roy Williams and Allen Dorfman, in the office of Sen. Cannon, promised that senator a bribe. Thereafter, Mr. O'Malley, Mr. Massa and Mr. Lombardo joined with Williams and Dorfman to carry out that bribery.

"By attempting to carry out the conspiracy of that bribery, they defrauded the [Teamsters union's] Central States Pension Fund."

Roller told the jurors they would be presented four kinds of evidence -- stipulation, witness testimony, documents and tape recordings.

The five are accused of offering Cannon, who was chairman of the Senate Commerce Committee, a lucrative Las Vegas land deal if he would scuttle trucking deregulation legislation.

The legislation passed, however, and Cannon, whom the prosecution will not call to testify, was not cited for any wrongdoing. Cannon, however, is on defense attorneys' list of possible witnesses.