The government's key witness in the bribery trial of former D.C. Superior Court Judge Robert H. Campbell testified yesterday that a local construction firm for which he worked provided liquor, topsoil and other favors to the jurist because he "was in a position to be lenient" in the handling of traffic tickets issued to the company.

Robert Payne Jenkins, once a high-ranking official of Excavation Construction Inc., also testified that he made a $2,500 loan to Campbell out of company funds in 1970, when Campbell was chief of the law enforcement section of the city prosecutor's office. That office has the authority to discount or dismiss traffic fines.

Jenkins testified that he arranged for the loan -- which has never been paid back -- after Campbell told him "he wanted to borrow some money." During a long day of questioning by Assistant U.S. Attorney John P. Hume, Jenkins also acknowledged that he got the former judge to sign a note for the loan at the request of the company's general manager, Larry A. Campbell (no relation to the judge), who agreed to allow the judge could borrow the money.

Jenkins testified that he held the note for five years but then returned it to Campbell, who by then was a judge on the local court, when he "asked for it back."

The former judge, Larry Campbell and Excavation Construction are being tried in U.S. District Court on charges of conspiracy, bribery and racketeering. The government contends that the former judge traded his judicial influence in exchange for more than $10,000 in cash, goods and favors from the Maryland-based construction firm.

Much of the government's case rests heavily on the testimony of Jenkins, who already has been convicted of lying to a grand jury investigating the Campbell case.

In its opening statement to the jury, the government said it would call Jenkins to testify as a hostile witness, describing him as still very much a "company man." Jenkins, who was once Larry Campbell's close assistant, was fired from his job at the construction company last fall.

The government's case focuses on overweight truck tickets issued to Excavation Construction dump trucks for violation of laws designed to protect city streets from damage. According to the prosecution, former judge Campbell imposed no fines on the company for more than 1,000 overweight truck tickets that he handled from 1975 to 1977.

Jenkins, described by the government as the company's trusted bag man, allegedly delivered seven payments to Campbell at that time. Jenkins, who spoke in a dry monotone throughout the day, spent much of his testimony recounting various favors that he said he had arranged for the judge.

Under questioning by Hume, Jenkins admitted that his credit card was used to pay for a rental truck that hauled the former judge's household furnishings from one house to the other in August 1975. Three years later, when the move was disclosed during a federal investigation, Jenkins said he spoke to the former judge, who told him "he had paid the men" who participated in the move.

Two of those men were called to the stand earlier in the trial, testifying that Campbell gave them $60, although the government argues that that would not be enough to cover the normal cost of the move or the salaries of the ECI employes who carried it out.

There also has been testimony, supported by Jenkins yesterday, that about $55 in drain work was done at Campbell's house by a private plumbing firm whose bill was paid by the construction company with Jenkins' approval. According to earlier testimony, an invoice in the company's records showed the work was done on company property, but the plumbing outfit's original invoice showed the work was done in Campbell's home. There also was testimony that the owner of the plumbing company is a personal friend of both Jenkins and Larry Campbell, the construction firm's general manager and co-owner.

Jenkins also testified that in September 1974 Campbell asked him to "get him some good prices on liquor" that he planned to use for a party. Jenkins testified he then went to a liquor store and placed an order for $913.73 for liquor, which was charged to a construction company account for "business promotion." There was earlier testimony that the former judge held a birthday party for his wife at L'Enfant Plaza at that time.

Just before Judge Thomas A. Flannery recessed the court proceedings yesterday, prosecutor Hume asked Jenkins if Campbell had ever asked him for money after the 1970 loan.

Jenkins said that the former judge had made such requests, saying he needed money to pay some bills. Jenkins said, however, that he could not pinpoint the specific times when such requests were made. When defense lawyers protested about the vagueness of Jenkins' response, Hume said he will augment Jenkins' testimony with documents and other evidence.

Hume is expected to continue his questioning of Jenkins today.