Robert Slater is a professional. When he heard the pope was going to visit the United States, he promptly stopped production of the "Re-Elect Carter" and "Kennedy in 1980" campaign buttons his 40-year-old Manhattan-based firm was making and ordered his 60 employes to "work strictly on the pope."

"Its the biggest event we've had since we did the Beatles buttons in 1964," said Slater, whose firm, N. G. Slater Corp., has already sold more than a million pope buttons, pennants, medallions and photographs to distributors in the six cities, including Washington, on the pope's itinerary.

Washington attorney Paul Toulouse, on the other hand, is an amateur in the souvenir business. A few weeks ago, he and a friend decided it "might be a fun idea" to design and sell a "tasteful" T-shirt. They invested $6,000 in 2,500 shirts, found a graphics designer to create the motif, and have been lining up vendors to sell them.

The economic impact of the pope's U.S. tour cannot be measured precisely, but there is no question that the marketplace is bustling. An unparalleled assortment of souvenirs, for instance, including video tape cassettes of papal visits and record albums featuring the papal voice, have jammed vendor stands here and elsewhere.

Merchants of memorabilia proclaim intentions of being "tasteful" about their commercial role in what is billed as a religious event. Nonetheless, the variety of pope-related sales activity has a uniquely American flavor.

"I don't see any sin in making money," said G. W. Smith, a Falls Church businessman who, under the name of "Mr. Malone," is selling gold-plated medallions of the pope with the aid of a squad of college students. Smith said his product, which sells wholesale for $12.95 and could fetch more than double that amount on the Mall, "offers the consumer a choice between cheap plastic items they will throw away and something they can pass on to their family."

Newspapers have prepared special features for the papal visit. The Boston Globe gave away full-color Vatican flags printed on slick-paper to those who bought the newspaper the day of the pope's visit.

Here, The Washington Post and The Washington Star will offer special papal commemorative sections as part of the Sunday newspaper. The papers will be sold on the Mall where the Pope's Sunday mass is expected to draw up to a million people.

In another medium, a New York company called Infinity Records has produced more than a million copies of a recording of a youth song festival taped in Poland last year that includes the pope singing six songs. If the album sells out, the pope might qualify for a Platinum Record award from the recording industry for topping the million mark.

"We don't like to use that phrase," said Infinity spokesman Bert Bogash. "We don't want to be confused with hucksterism. We are not in this to commercialize the pope's visit." In keeping with that goal, Bogash said, an unspecified portion of the proceeds will be donated to the National Conference of Christians and Jews and to the Pontifical Mission Society of New York.

In Boston, The Pontifical Mission Society has exclusive rights to the record there until Oct. 28. A company spokesman, who refused to identify himself, shouted angrily into the telephone that the enterprise was not designed to "make money."

Most of the manufacturers or distributors interviewed conceded that they had prepared themselves for accusations that they were exploiting the papal visit for crude commercial purposes.

"I really anticipated (criticism)," said Slater, "But I take more flak about pro- and anti- abortion or pro- or antigay buttons. I have letters on my desk from the (New York) archdiocese asking for samples of the items to purchase. If the Catholic Church itself is not giving any flak, I don't think . . . that people are up in arms looking to nail anyone for it."

Slater donated 250 buttons and 250 pennants to a parochial school in Harlem. He said the nun he spoke to about the donations wanted to know if "there were any strings" attached.

"In New York," he mused, "even the sisters are hip."

The Washington archdiocese endorsed only a small portion of the memorabilia, the $37,000 worth of flags, coins, medallions and pictures now being sold through area churches and schools in an effort to discourage the selling of souvenirs on the street.

In Boston, police arrested 40 vendors during the pope's visit Monday after the City Council warned that a seldom-used ordinance banning vendors from most areas of the city would be vigorously enforced during the visit.

"It was felt that vendors walking around would cause problems for people there, would obstruct sight and would detract from the dignity of the event," said Peter Woolchuk, a city spokesman.

In Washington, officials have banned private souvenir and food vendors from their normal sites on Constitution and Independence avenues. However, 130 private vending sites have been designated on side streets as well as on the sidewalks on Pennsylvania Avenue.

In Omaha, Neb., 125 miles from the Des Moines sites the pope visited yesterday, John O'Donnell ordered a batch of 5,000 "humorous" bumper stickers.

"They express a unique feeling that many will have when they see the pope," he said, "a quiet, religious feeling inside, that they'll want to express with a bumper sticker that says 'God Bless John Paul II.'" Other bumper stickers O'Donnell offers include "Pope John Paul Blessed My Car," "John Paul II, Do I have a Confession for you," and "John Paul II Is My Kind of Polack."

There are some entrepreneurs who are slightly off the beaten track. In Norristown, Pa., for example, Anthony Castenova has a pewter plate embossed with a replica of the centennial seal from 1876, surrounded by pictures of the pope, Ulysses S. Grant, George Washington, Jerry Ford and Jimmy Carter that he hopes will be given to the pope. Castenova, a 29-year-old truck driver, hopes to sell replicas of the plate commercially. He calls it "The Miracle Souvenir."

And Veronica Lueken, of Queens, N.Y., self-proclaimed "Seer of Bayside," traveled to Boston this week with some 100 followers. She said she has personally seen the Virgin Mary several times, and, for an unspecified donation will offer for sale laminated rose petals or a medallion, both of which possess, she said, the twin virtues of "cure and conversion."

Either one, she said, will cure illness and convert metal into gold.