During his campaign for the sheriff's office two years ago, Donald Lacy promised to shake the dust from Loudoun County's deputies and mold the tobacco-spitting department into a modern police force.

The voters were impressed. Lacy, a 32-year-old former deputy, stock clerk and gas station attendant, upset Democratic incumbent Robert Legard, who had held the job for 16 years. Some of Loudoun's deputies, however, were less enamored with Lacy, who became the first Republican sheriff in Loudoun history.

Now, 21 months after taking office, Sheriff Lacy has been accused by some of those deputies of misappropriating funds, unauthorized absenteeism and sexual improprieties.

Rumors about the investigation have spread like a hay fire across this county, which begins as suburban sprawl 30 miles west of Washington and continues toward the Blue Ridge mountains until only an occasional crossroads town interrupts the hilly pattern of farms and forest.

"In politics you can't say anything worse," says John Moorcones, a Leesburg lawyer, friend and political adviser to Lacy. Earlier this week Moorcones called the allegations "the foulest rumors I have ever heard about a politician. I've gone through some foul campaigns, but these are just extremely malicious rumors and they're not true."

A special prosecutor and the Virginia state police are now investigating the allegations, which were contained in a letter from the county commonwealth's attorney to a Loudoun circuit court judge last week.

Lacy, a father of five who is married to a physician's assistant, has handed over the running of his office to Capt. Albert Keyes during the inquiry. He also has resigned as finance chairman for Republican Kenneth Rollins, the mayor of Leesburg, who is running this fall for the county's seat in the House of Delegates.

"I don't know what to make of all this," Lacy said one afternoon last week, standing in front of his Sterling Park brick rambler, his hair uncombed and his face unshaved. "Each day I hear something new. It's politics."

There are a numbing variety of allegations and denials offered by friends and enemies of Lacy. Those who say he is guilty are themselves accused of committing the lowest form of political smear. Lacy defenders say he has been sabotaged by politics, others blame deputies who resented his campaign -- especially after he refused to check some of the more controversial promises at the jailhouse door.

"I do not believe that it is merely coincidental that all this is breaking loose 30 days before an election in which he is serving as a cochairman for one of the candidates," the Rev. Charles D. Grant, the sheriff's departmental chaplain and a close friend of Lacy's, said in a handwritten statement. "I am personally convinced that he is the victim of vicious lies and rumors which have been started by persons who in some cases are extremely jealous of how successful and effective he has been as sheriff . . . . "

Rollins said, however, that he knew nothing about the allegations, and did not believe they were politically motivated. "I don't think there's any connection with my campaign and the sheriff," said the mayor, who is challenging Democratic Del. Earl E. Bell.

"It has had its effect," said a lone woman volunteer at the county's Republican headquarters in Leesburg. "They have done what they set out to do."

Grant, the principal of a Christian school in Loudoun as well as vice president of a small printing company there, says he was approached on Sept. 28 by one of Lacy's deputies who divulged the allegations, including accusations of sexual misconduct. Grant said the deputy, who claimed to be spokesman for a group of deputies, promised the charges would be withdrawn if Grant could persuade Lacy to resign.

"I was to be the purveyor of their blackmail attempt," said Grant, 45, who regularly accompanies deputies on night patrols. "I wanted to know specifically what was the basis for the charges. He the deputy did give me some specifics. But when I went about myself to contact some of the people mentioned, in every case they denied it."

Thomas D. Horne, Loudoun's commonwealth's attorney, listed the allegations in a letter last week to Circuit Court Judge Carlton Penn. Penn selected Richmond Commonwealth's Attorney Aubrey M. Davis as a special prosecutor and directed him to investigate "allegations of misfeasance, nonfeasance, malfeasance or criminal activity" inolving Lacy.

Neither Horne nor Davis would release the details of the allegations. Davis did say that "in light of the allegations and out of fairness to the sheriff and the public they ought to be fully investigated and cleared up."

Horne's letter went to the judge a day before Lacy fired his chief investigator, Jonathan M. Sheldon, who had worked as a deputy since 1969 when he was hired by Sheriff Legard. Sheldon released a statement that quoted portions of the termination letter from Lacy.

In that letter, Sheldon said, he was informed that "while your derogatory remarks about other officers and myself may not be criminal, they are in direct violation of departmental policies and procedures concerning 'unbecoming conduct' and 'public statements and appearances.' "

Sheldon has filed a grievance against the sheriff's department, accusing it of firing him without cause. He has refused to comment, citing the advice of his lawyer.

Carl Henrickson, the Democratic chairman of the Loudoun Board of Supervisors, said the allegations he has heard do not match his impression of Lacy. "Don Lacy cares a lot about what he's doing, and he's done a genuinely good job. But I think he tends to be frustrated by the political process," Henrickson said.

"He's more reserved, not what I could consider a typical politician," says Arlington Sheriff Jim Gondles, a Democrat. "He's big on undercover drugs. He's always talking about undercover investigations."

Lacy's critics say he is overly ambitious. Even some friends concede that the sheriff is sometimes harsh with his deputies, loudly berating them for jobs he considers poorly done. But friends and enemies agree that the rookie politician plotted a masterly campaign for sheriff in 1979.

Lacy built his campaign on two promises: He would expand and modernize the force and increase police coverage in the eastern half of the county where the population was most dense and growing.

Lacy successfully converted the long-term animosity between suburban newcomers in the eastern half of the county and old-timers in the west who still dominated county politics, to defeat Legard. His campaign was also helped by a 1977 report from a special Loudoun grand jury that found the administration of the sheriff's department to be "inadequate and inefficient."

True to his campaign promises, Lacy opened a substation in Sterling Park. and increased the department from 68 to 99 officers. But in reorganization, some veteran deputies were passed over for promotions.

"I am 100 percent convinced that the sheriff is the object of a vendetta," Grant said. "He has been crucified. He has been crushed."