Charles S. Robb and J. Marshall Coleman, in an unusually negative and stinging round of allegations in their campaigns for Virginia governor, have taken to the airwaves to accuse each other of being soft on drugs.

Democrat Robb started it last week with a 60-second radio commercial that quotes Coleman, the Republican, as saying "marijuana is no longer a serious criminal problem." The ads contend that as state attorney general Coleman drastically cut back support for local law enforcement and conclude: "Chuck Robb knows that drugs are a big problem in Virginia. Marshall Coleman is not so sure."

During the weekend, Coleman struck back with a 60-second radio spot outlining Coleman's antidrug record and poking fun at Robb, suggesting that the Democrat's charges are "enough to make you wonder what Robb's been smoking." The ad asserts that Coleman pushed for tougher sentencing for drug smugglers "while Robb did nothing."

The GOP spot concludes: "Chuck Robb has no law-enforcement experience at all. And apparently not much of a campaign either."

The two ads, which are the first negative media spots either candidate has run so far, suggest both sides may be preparing to devote a portion of their million-dollar media campaigns to attacking their opponent.

The Robb spot, which ran in media markets across Virginia, made Coleman and his campaign aides so angry that they rushed to the airwaves with their own ad in order, they say, to set the record straight. "A blatant distortion of Marshall's record," charged Coleman campaign manager Anson Franklin, who said his candidate was furious after hearing the Robb ad on a Richmond station last week.

Robb himself was upset with Coleman's rebuttal. "Anything in such poor taste would be something people would see through and assess the credibility of a campaign that would pay for that," Robb said through a spokesman.

However, Robb's aides professed to be delighted by the virulence of Coleman's response, saying they believe they've hit a nerve in the usually cool and unflappable Republican and that his reaction may cost him votes.

"It's a classic case of overreaction," said Robb campaign spokesman George Stoddart. "My guess is if I put the matter to a vote before the Robb campaign steering committee, they'd be more than willing to pay for running it Coleman's ad from now until election day," said Robert Squier, Robb's media consultant and the author of the Robb spot.

Robb and Coleman, in statements to Common Cause of Virginia last month, both pledged to focus their campaigns "on the issues and to avoid unsupported personal attacks on my opponent." But on the campaign trail, Coleman has taken to labeling the conservative Robb as "a son of the Great Society" because of his marriage to late President Lyndon Johnson's daughter, while Robb and a series of campaign surrogates have painted Coleman as irresponsible and untrustworthy.

The Robb radio spot quotes one of those surrogates, retired House Appropriations chairman W. Roy Smith, praising Robb for favoring stiffer sentences for drug dealers, then citing Coleman's statement playing down marijuana. Smith heads "Virginians for Robb," a group of conservatives, some of whom, like himself, have supported Republicans in the past but find Coleman not to their liking.

The Coleman marijuana statement comes from the last paragraph of a Richmond News Leader article in June, 1978, on a Coleman press conference. It is a paraphrase, not a direct quote, and campaign spokesman David Blee said Coleman denies making the statement.

But the reporter who wrote the article, David Lowery, now with the Cincinnati Enquirer, said today, "I have no doubt at all that he said that. If he hadn't, why did they wait until now to challenge it?"

The Robb spot also accuses Coleman of cutting back on support for the attorney general's technical assistance unit, which advises local prosecutors on legal matters. Franklin, who was Coleman's chief administrator as attorney general, said the unit's lawyers were reassigned to handle more pressing criminal cases.

Coleman's ads emphasize his support for uniform sentencing, which he claims will be tougher on drug pushers, and cites his efforts to fight the early release by a Southside Virginia judge of three convicted marijuana smugglers.