When trying to sell soap, television advertising experts say, don't tell viewers, that other brands of soap are rotten. The experts say you should tell viewers that your soap is best - make people feel good about your product.

Selling candidates for Congress over television works much the same way, according to political advertising advisers.

But if their current commercials are any indication. Republican John F. Herrity and incumbent Democrat Herbert E. Harris II, opponents in Northern Virginia's hotly contested 8th Congressional District, do not subscribe to the soap-selling theory of political advertising.

"What they're doing is a very dangerous business, said a political consultant with 10 years' experience in Northern Virginia.

Herrity's latest commercial, which was aired over the three network affiliated stations in Washington on Thursday and yesterday, was aptly labeled by the people who filmed it: "Direct Mail Attack."

A graying actor in a dark suit appeared in the commercial and condemned Harris for mailing "junkmail" for political gain, mail that is "not informative as the law requires" and abuses the congressional franking privilege.

One of Harris' commercials brings up the Northern Virginia water shortage of the summer of 1977, pictures a little boy trying to get water from a tap that runs dry and has a voice-over that says: "Remember when Northern Virginia had a water shortage and Jack Herrity's solution was to use less water?"

Before election day on Nov. 7, Herrity plans to spend nearly $70,000 on television advertising, according to his media adviser Norman Bishop. Harris plans to spend more than $25,000, his media adviser.Arnold Bennett said.

The campaign spending on negative-sounding commercials, which marks the first time two-term Rep. Harris has felt the need to buy television time, may backfire, according to some media consultants.

"People who look at TV ads are jaundiced in the first place. The more a candidate excoriates his opponent, the more he loses credibility," said Stephan Lesher, executive vice president of Rafshoon Communications, the company that made Jimmy Carter's television ads.

Lesher, a veteran of three successful congressional elections as a media adviser in Indiana, said that "if you begin name-calling on television, the voter tends to dismiss both candidates."

Television works best, according to media adviser to Republican candidates in Virginia, "when it is a feeling that you convey, not a specific issue.

"Negative ads generally cause a negative feeling about the entire race and both candidates."

The media adviser, who asked that his name not be used, negative commericals are particularly harmful to Herrity, who is chairman of the Fairfax County board of supervisors, and Harris, "because the image they already have is that they are always fighting with somebody."

Not all of Herrity's commercials have been negative. Four 30-second spots aired immediately after the District in Columbia's primary election in September showed Herrity walking through the woods and saying, "I can make things happen."

The bland-to-bitter transition in advertising is just part of the growing enmity in the race between the two well-known Northern Virginia politicians who, according to a long-time political observer, "don't like each other ver much."

The vituperative tone of the TV ads also popped up this week in three debates between Herrity and Harris. In one of those debates, Herrity became angry and said he is "ashamed" to have attended the same law school (Georgetown University) as Harris.

The ill-feeling generated by the debates and the commercials may trigger more such advertising, according to Bishop, of Bishop and Bryant Inc., an Alexandria firm handling Herrity's campaign.

"We feel there are some very legitimate negatives in terms of Herb Harris that need to be brought out," said Bishop.

The Herrity commercial attacking Harris for allegedly putting campaign propaganda in $50,000 worth of newsletters to constitutents gets at one of those "negatives," Bishop said. (Harris defends the newsletters, saying that he believes in "communication" and in accordance with House rules, stopped the mailings 60 days before the election.)

Harris, in what may be a first for a Washington-area congressional race, goes after votes in one of his ads by saying that he has always been a supporter of the Metro subway system while Herrity has dragged his heels.

A long-time supporter of Metro, Harris made the commercial, in part, to cash in on public support for the subway system in Northern Virginia, according to his media adviser Bennett.

But one effect of the commercial has been to prompt angry protests from Herrity.

"I've never tried to cut Metro short in Virginia," Herrity said. What he has done, he said, is to insure that taxpayers do not "blindly accept the largest public works system in the history of the world."

While Herrity plans to make commercials to respond to Harris' paid ads, Harris' media expert said Harris has neither the money nor the interest to respond to Herrity in kind.

"We have filmed all the commercials we are going to do (a total of seven) and we are not going to make any response to Herrity's attacks," Bennett said.

"We don't believe in a lot of hype."