Keith Conquest doesn't need a public opinion poll to know that Olliemania is fizzling.

Conquest has a lot of evidence at his Fit to a Tee Shirt store in the Georgetown Park shopping mall that support for charismatic Marine Lt. Col. Oliver L. North has waned: stacks of unsold T-shirts carrying the former presidential aide's picture.

In the days immediately after North testified before Congress in July, Conquest's store couldn't keep North T-shirts in stock. Demand was so great that "they were blowing out of the store," Conquest recalled.

But sagging T-shirt sales are only one index to North's fading popularity. A decrease in pro-North letters to members of Congress, disappointing sales of an "instant" paperback and videotapes of his testimony, and an opinion poll released yesterday indicate that North's popularity has declined as sharply as it rose.

Some of North's strongest supporters said they expected as much.

"That's standard . . . a matter of news exposure," said Utah political consultant H. Keith Haines. He helped organize a defense fund that has raised $1.5 million for North, who is expected to be indicted on charges stemming from his role in the Iran-contra affair.

In the poll, a national telephone survey by Louis Harris, 65 percent said they think North was "more wrong than right" by secretly selling arms to Iran and diverting proceeds to the Nicaraguan contras. Seventy-three percent said they think that North likely committed a crime and 68 percent said they support prosecution of anyone suspected of violating the law in the Iran-contra affair.

The pollsters also found that 84 percent believe that North's superior at the National Security Council, Rear Adm. John M. Poindexter, also may have violated the law.

While North's supporters dispute the significance of his declining popularity, there appears to be little disagreement among many merchants and conservative politicians that he was good for business, at least briefly.

"We knew it would peak and that that falloff would be quite rapid," said Ann Maitland, a vice president of Pocket Books, which rushed a 753-page paperback edition of North's testimony to the market three days after his last appearance on Capitol Hill.

The company printed 775,000 books but expects to sell only half of them, according to Irwyn Applebaum, the firm's president and publisher.

Sales of a 90-minute videotape titled "Oliver North: Memo to History" have been "very, very disappointing," said Jaffer Ali, vice president of sales for MPI Home Video of Chicago, which produced 100,000 of the $19.95 tapes, half of which are unsold.

"North made some very stirring patriotic comments, but people are voting with their pocketbooks," Ali said.

Nevertheless, Pocket Books and MPI said they made money on their North releases. The book was a "modest blockbuster" rather than the "huge blockbuster" that the publisher expected, Applebaum said. The first of two North biographies reached bookstores yesterday, and their publishers said good sales are expected.

Turner Broadcasting Co. of Atlanta produced a two-hour, $24.98 videotape of North's testimony and quickly sold 36,000 copies, enough to qualify it as a "golden video" and meet the company's sales projections, according to Steve Chamberlin, vice president of Turner Home Entertainment.

Sales have slowed, and Chamberlin said he expects to sell about 10,000 more tapes, but with more difficulty. "You have quick-cycle effect" with such "video instant celebrities," he said.

Sales of North T-shirts and bumper stickers peaked quickly here after his testimony, according to several merchants and souvenir shop operators.

Dallas Alice Inc. of Rockville produced and sold more than 13,000 shirts bearing a cartoon of North testifying under the legend "Ollie's Follies." "I'm surprised it's still selling," said Barney Harris, the firm's general manager.

One key to the sales was quick production of a shirt that did not take sides on North's testimony, Harris said. The rule of T-shirt sales is "If it's topical and he's in the limelight, it sells," she said.

The Republican National Committee and the National Republican Congressional Committee (NRCC) launched mailings in late July in attempts to cash in on North's support of the contras. Democrats and some Republicans attacked the effort, but spokesmen for both groups said yesterday that they were financial successes.

Neither group would discuss the amount produced by their mailings. The NRCC request by Rep. Guy Vander Jagt (R-Mich.) under the name "GOP/North Victory Fund" will result in a donation of "at least $50,000" to North's defense fund, a committee official said.

Congressional mail, which strongly supported North as his testimony began, decreased later in the hearings, according to the staff members of the investigating committees.

Mail received by Senate panel Chairman Daniel K. Inouye (D-Hawaii) initially was pro-North, then began turning against him during the week he testified and has remained 60 percent to 70 percent against him, according to Gregg Takayama, an Inouye spokesman.

During the testimony, Sen. Orrin G. Hatch (R-Utah), a conservative who defended many of North's statements, received about 500 letters daily, most of them supporting North. The number has dropped to about 10 a day, and most still voice support, said Paul Smith, a spokesman for Hatch.

Several conservative organizations in Washington apparently believe that North's name is still good for fund raising. The American Freedom Coalition, a Washington group raising funds under the name "Emergency Project to Support Colonel North's Freedom Fight in Central America," and the Nicaraguan Development Council Inc. of Washington last week made mailings that invoked North's name.

The Freedom Coalition offered a free videotape of North's testimony to contributors of $25 or more. Adolfo Calero, the member of the contra political directorate who signed the Development Council's "Urgentgram," described North as "a man I am proud to call my friend."