An investigation into conflict-of-interest allegations against Fairfax County Board Chairman John F. Herrity that has sharply divided county political and civic leaders is drawing to a close after five weeks, according to the county's chief prosecutor.

Regardless of whether Commonwealth's Attorney Robert F. Horan Jr. decides to prosecute the Republican board chairman, many political observers say the probe is likely to tarnish Herrity and the Board of Supervisors.

"All the members of the board are very concerned about the fallout from this," said board Vice Chairman Martha V. Pennino (D-Centreville). "When you deal with developers, you have to stay squeaky clean to develop confidence among the constituents. When one member goofs, it affects all of us. People have a tendency to put us all in the same boat."

The probe centers on Herrity's participation in the county's review of his business partner's land rezoning application. It includes allegations that Herrity violated county law when he failed to disclose publicly campaign contributions from principals involved in other development cases before he voted on them.

Herrity, 54, first elected to the county board 10 years ago, has made no public comment on the investigation for three weeks and declined to be interviewed last week. He has maintained that any violations of state or county law he may have committed were unintentional.

Herrity has said his apparent failure to adhere to county and state public disclosure laws was chiefly the fault of developers who had not alerted him to the potential conflicts in the cases he reviewed.

Throughout the investigation, Herrity has retained the support of many of the county's business leaders and corporate officials.

James P. Popino, president of the Fairfax County Chamber of Commerce, called Herrity "the best chairman of the board we've had" and said, "I don't think anything I've seen so far does anything to cause me to reevaluate that opinion . . . . I'd hate to see a career ruined because of one or two mistakes."

State Sen. Wiley F. Mitchell Jr. (R-Alexandria) also defended Herrity, saying that he "can't help but feel a twinge of sympathy for someone who gets tangled up in complex conflict law. I hope that most of the people of Fairfax County will see this for what it is, an inadvertent failure to follow some very technical" legal requirements.

Under Virginia law, violations of the conflict-of-interest provisions are a criminal misdemeanor and carry penalties of up to a year in prison and/or a maximum $1,000 fine. Although the law allows a judge or jury to remove a convicted official from office, such action has been rare in the state courts.

Supporters and critics of Herrity have viewed the probe with increasing seriousness as it has stretched from what was expected to be a brief inquiry into a lengthening investigation by prosecutors.

"There's a lot of sympathy for Jack, but I think that sympathy has become less and less pronounced as more things have come out," said Pennino, who has maintained a close and cordial working relationship with Herrity and who defended him early in the investigation. "In the beginning, it looked like he had goofed. But how many goofs are forgivable? The question most often asked now is, 'How is Jack going to get out of this?' "

Pennino said letters, phone calls and conversations with her constituents show declining support for Herrity's explanations.

The probe has had an impact on the special election for the vacant Providence district seat on the county board. The Republican candidate, state Del. Stephen E. Gordy of Fairfax, said he has not solicited Herrity's support because the chairman's involvement could be a political liability.

"Even though our system is such that people are innocent until proven guilty, I just think it would be better that he not participate in the campaign," Gordy said. "Jack has steered very clear of my race. That's the way I've wanted it to be."

Whether or not Horan decides to prosecute Herrity, county officials and other political observers predict that the investigation could leave lasting political imprints on Herrity and, to a lesser extent, on other Fairfax supervisors. Board members will face the voters again in the November 1987 election.

"I don't know whether this will blow over or whether it will hang around his Herrity's neck when he runs next year," said William W. Hanks, president of the Fairfax County Federation of Citizen Associations. "It's too early to say. It all depends on what Mr. Horan says."

Hanks, who said his comments were based on discussions with members of his organization, said that "the whole thing will be a minus for Herrity. But it will be a small minus if the case goes to trial and he is found innocent."

The investigation centers on Herrity's participation in the county board's four-month review of a rezoning case involving Hersand Builders Inc., a Fairfax firm seeking the county's approval to build a town house development in the Springfield district.

Herrity failed to disclose that he owned a substantial interest in a Fairfax City condominium venture with Hersand's president, Herbert L. Aman III.

Under Virginia law, local officials are required to disclose whether they share a business or financial relationship with anyone connected with a land use proposal and then must disqualify themselves from the case.

Horan also is investigating Herrity's failure to make a public disclosure before voting on three additional development proposals during the past year involving individuals who contributed to his 1983 reelection campaign.

Fairfax County law requires supervisors, before considering a land use application, to disclose publicly whether they have business or financial ties with anyone involved in those proposals or whether they received gifts or donations exceeding $50 from them during the preceding five years.

The penalty for violating the county law is a maximum six-month jail term and/or a fine up to $500