Fairfax County prosecutor Robert F. Horan Jr. called on Virginia state Attorney General J. Marshall Coleman yesterday to step aside and appoint a special investigator to probe conflict-of-interest allegations involving a highway commissioner who is helping raise funds for Coleman's gubernatorial campaign.

"I don't see how in heaven's name he Coleman can expect the public to accept the fact that he's going to investigate people who are contributing to his campaign," said Horan, a Democrat.

Lt. Gov. Charles S. Robb, the Democratic nominee for governor, and Andrew P. Miller, Coleman's predecessor as attorney general, said separately that Coleman's promised inquiry illustrates the need for him to resign from his state office while he is the Republican nominee for governor.

The Democrats were reacting to an announcement by the attorney general's office that it is reviewing allegations that William B. Wrench, the Northern Virginia member of the highway commission and a member of Coleman's campaign finance committee, may have acted improperly in successfully urging rerouting of a proposed major highway closer to some of his Fairfax County properties.

Miller, now a Washington lawyer, went further than Horan, saying Coleman should resign as attorney general, as Miller did four years ago when he unsuccessfully sought the Democratic nomination for governor.

"Mr. Coleman is clearly barred from rendering any opinions" in the matter, Miller said, because both Wrench and Wrench's attorney, John T. Hazel, who would also benefit from the new highway alignment, are members of Coleman's 100-member finance committee. The committee, Miller noted, is giving expense money to Coleman because he is accepting only half his state salary during the campaign.

Robb said yesterday that if he were in Coleman's position he would resign, but he stopped short of calling for Coleman to do so. Many Virginia Democrats contend that it is traditional for the state's attorney general to resign during a campaign for higher office because his position, unlike that of lieutenant governor, is a full-time job.

At an Alexandria news conference, Horan said his concerns about the Coleman investigation were "above and beyond the question of whether they're guilty of any wrongdoing. I'm not even going into that. It's on the mere question of whether or not one can legitimately investigate people you're financially beholden to," Horan said as he announced formation of a prosecutors group backing Robb.

Earlier this year, Coleman said he returned campaign contributions from persons who were under investigation by his office. But yesterday his campaign spokesman David Blee declined to say how much money Wrench and Hazel have given to the campaign.

But both Blee and Ben Ragsdale Jr., a spokesman for the attorney general's office, attempted to draw a distinction between an "investigation" and what they said was Coleman's "review" of Wrench's vote on the controversial Springfield Bypass.

"I personally don't call this an investigation," Ragsdale said. "An investigation is when you bring the state police in. This is a serious review or examination."

Ragsdale said he expects Coleman will get a report on Wrench "within a few days."

Another former Democratic attorney general, Anthony Troy, criticized Coleman's pledge to return contributions from persons under investigation. "If a person or business suspects they are being investigated by the attorney general's office," Troy said, "all they need to do to confirm that suspicion is contribute to Mr. Coleman's campaign. If the contribution is returned, then the integrity of the investigation is betrayed."

Wrench, who represents the Washington suburbs on the 10-member state highway commission, offered the motion last month that moved the proposed road closer to three of his land holdings, and put an interstate cloverleaf on Hazel's property.

The attorney general's inquiry will focus on whether Wrench's vote was illegal under the state's conflict-of-interest law.