The bribery trial of former D.C. Superior Court Judge Robert H. Campbell opened yesterday in U.S. District Court with defense lawyers attacking the testimony of the government's star witness as the "mutterings of a liar" who used some of the alleged bribe money to support an illicit love affair.

Campbell, 56, is accused of accepting more than $10,000 in cash and gifts in return for dismissing hundreds of thousands of dollars in traffic tickets for Excavation Construction, Inc. of Bladensburg.

The government's case rests largely on the testimony of Robert P. Jenkins, a former employe of the firm, who the government contends made more than half a dozen payoffs to Campbell.

Defense lawyers told the jury of eight women and four men that Jenkins, a one-time high ranking employe with access to the firm's checking accounts, withdrew company funds but kept the money for himself.

They said Campbell paid Jenkins cash for several jobs and favors performed for the judge, but no receipts were given. Jenkins, they said, used some of the money drawn on company accounts to support an extramarital affair with a woman the lawyers did not name.

Campbell and two codefendants -- the firm and its general manager, Larry A. Campbell (who is no relation to the judge) -- are charged with bribery, conspiracy and racketeering in what is believed to be the first criminal trial of a local judge here stemming from administation of the judge's official duties.

Assistant U.S. Attorney Carol E. Bruce told a U.S. District Court jury yesterday that the government's evidence would tell an "ugly story of the corruption of a judge." She said, "Personal and corporate greed is what this case is all about."

The former judge's lawyer, R. Kenneth Mundy, told the jury that as a judge, Campbell "never forgot he was first and foremost a human being."

Mundy said that Campbell, who was "raised at a time when being black was anything but beautiful and being poor was especially ugly," may have been motivated in his actions by the experience of watching white police officers "pouncing on the black drivers" for the construction firm and giving them tickets while letting other trucks pass by.

Mundy portrayed Campbell's handling of the construction firm's traffic tickets as a standard practice in Superior Court at the time.Judges routinely discounted and dismissed fines at their own discretion, he said.

Arnold M. Weiner, who represents Larry Campbell, said that defense evidence would show that the former judge imposed ticket fines on the construction company that would have been considered "on the high side" by traffic court standards. Moreover, Weiner said, the evidence would show that in one instance the judge imposed a high fine six weeks after he was alleged to have accepted a bribe.

Campbell unexpectedly retired from the local bench on full medical disability in December 1978. At that time he was under investigation by a federal grand jury, which returned an indictment against the former judge and his codefendants in August 1980.

The charges against the former judge, Larry Campbell and the construction company center on more than 1,000 traffic tickets issued to Excavation Construction drivers for violation of city laws that regulate overweight trucks.

Bruce, representing the government along with Assistant U.S. Attorney John P. Hume, told the jury that Larry Campbell knew "it was cheaper . . . to bribe a judge than to obey the law."

Bruce said the government's evidence would show that the construction firm saved $100,000 by not buying special use permits that are issued by the city government to trucks carrying overweight loads.

Although Larry Campbell knew his drivers would get tickets for not having those permits, Bruce said the government would prove he knew his company would not have to pay the fines "because the fix was in with the judge."

The prosecutor told the jury the government's evidence would show that from February 1975 to May 1977 "the corporation got off scot-free in over 1,000 cases" involving overweight truck tickets.

Bruce said the company "hoarded" its tickets until they knew that Judge Campbell would be presiding in court. Campbell, Bruce told the jury, not only informed the company when he would be assigned to hear traffic cases, but also once took 95 construction firm traffic cases while he was assigned to hear cases in the juvenile court.

While Campbell took a "tough and often arrogant approach to the citizens who appeared before him" in traffic court, Bruce said, the government would show he gave "startling and favorable" treatment to Excavation Construction cases.

That treatment gave the company a "tremendous competitive advantage" over other firms, Bruce told the jury. Their trucks could haul more dirt, make more trips and save fuel while knowing they did not have to pay for any overweight violations.

Jenkins, the government's key witness, was described by Bruce as Larry Campbell's "right hand man." In December 1979 he was convicted of perjury for lying to the grand jury investigating the Campbell case.

Jenkins subsequently began to cooperate with the government, but Bruce acknowledged yesterday that Jenkins is still a "company man" and will be called to testify for the government as a hostile witness. The trial is expected to last three weeks.