Virginia State Attorney General-elect J. Marshall Coleman has decided to discharge the senior deputy in the attorney general's office - a move one ranking legislator said will worsen the young Republican's relations with the Democratic controlled General Assembly.

Coleman said yesterday he will not reappoint Reno S. Harp III, described as a strong law and order advocate who has headed the attorney general's criminal division for the past seven years. Harp, has been on the attorney General's staff since he graduated from law school 21 years ago. And has become something of an institution in Virginia legal circles.

However, Coleman, 38, who will become the first Republican attorney general in Virginia history when he takes office Jan. 14, said in an interview he wanted people in policy-making positions on his staff whom he could be "comfortable with." Coleman said he decided against reappointing Harp on those grounds.

"It was a subjective decision . . ." Coleman said. "It is not a commentary on Harp's capabilities as a lawyer and it was not a political decision."

However Senate Majority Leader Adelard L. Brault (D-Fairfax) cautioned last night that Coleman's decision mayhave adverse political consequences for the incoming attorney general. Democratic legislators already have been eyeing him cautiously as the GOP's front-running candidate for governor in 1981 and some have predicted that he will have "no honeymoon" with the Assembly.

"You've heard of an unbridgeable gulf," Brault said last night. "There was certain to be a gulf between the attorney general's office and the legislature. I suspect this will tend to widen that gulf. Whether or not it is a bridgeable gulf is something we will have to see."

Before his surprisingly easy victory in November over Demoncrat Edward E. Lane, Coleman had promised he would not make wholesale changes in the state's 90-member staff of lawyers. All the lawyers are subject to appointment by the attorney general and serve at his pleasure.

Yesterday, as Coleman disclosed the Harp decision, he said he would not be making any other immediate top level changes in his staff but said he could not rule them out at some time in the future.

He said that he had also decided against re-appointing August Wallmeyer, 27, a special assistant to the current attorney general Anthony F. Troy, Wallmeyer, who serves as the attorney general's public relations man, had been in the office less than a year and he said yesterday Coleman's action was "not unexpected in my case."

Harp, who was first named an assistant attorney general in 1956 under then attorney general (later governor) J. Lindsay Almond Jr., is currently one of the five deputy attorneys general on Troy's staff. In serving Troy and his precedessors Harp has developed a reputation as a hard-line opponent of moves that he said would handicap police in dealing with criminal suspects.

Because of Harp's long tenure, Coleman's decision caught some legislators by surprise. Harp "was for all practical purposes a career employee," Brault said. "He had a statewide reputation as an expert on criminal procedure and this action will be a severe loss to the commonwealth."

Coleman said last night he hasn't picked "anybody yet" to succeed Harp in the position, which pays between $30,000 and $32,000 a year.

Coleman declined to be more specific as to why he would be uncomfortable with Harp. During his campaign Coleman spoke out frequently on the need to improve Virginia's criminal justice procedures but never directly attacked Harp during the race.

In addition to handling appeals on procedural points in criminal law, Harp's division is also responsible for resisting the hundreds of court petitions filed by state prison inmates as well as enforcing the state's new anti-trust law against combinations.