An American delegation was scheduled to arrive in Paris today to begin what Commerce Department officials describe as some of the toughest diplomatic talks of the year: trying to persuade the Bureau of International Exhibitions to pick Chicago as the site for the 1992 World's Fair.

The Windy City has been lobbying for the fair for three years and has developed a $1 billion package that the BIE's staff describes as "a textbook example" of what potential host cities should do. Even so, Commerce officials are nervous.

That's because Seville, Spain, also wants to host the fair that will mark the 500th anniversary of the explorations of Christopher Columbus. And in the past few weeks, the BIE has received requests from Mexico and 13 other Latin American countries to join the 54-year-old treaty organization. If accepted, they are expected to vote for Seville.

And because the United States has been the only member in recent years to encourage the Latin American countries to join the BIE, it may have trouble putting up a fuss.

That's just one of the problems facing the delegation, which is headed by Charles Z. Wick, director of the U.S. Information Agency, and William H. Morris Jr., assistant commerce secretary for trade development.

"Getting Chicago chosen is going to be difficult," said Dr. George L.B. Pratt, director of Commerce's International Expositions Office and chief adviser to the U.S. delegation. His office, which has a full-time staff of four, is responsible for all world's fairs held in the United States. It's a tougher job than most might think, said Jerome S. Morse, Pratt's deputy.

Earlier this year, the office had to mediate a battle after Miami heard that Chicago was planning to host the 1992 fair. Miami argued that its large Hispanic community and Florida's history gave it closer ties to Columbus -- who sailed under Spain's sponsorship -- than Chicago had.

Chicago called in Sen. Charles H. Percy (R-Ill.) and Rep. Dan Rostenkowski (D-Ill.) to make its case before the office, while Miami hired Hill & Knowlton, the international public relations firm. The heated dispute ended a few weeks ago when President Reagan accepted Pratt's recommendation that Chicago be chosen.

(Miami reacted by saying it still might hold its own world's fair to commemorate Columbus. That quickly brought a warning from Pratt, who said BIE rules prohibit member countries from allowing anyone to invite foreign countries to take part in a non-sanctioned world's fair. "We just couldn't allow them to violate the treaty which we signed," he said.)

The BIE sanctions two types of world's fairs: universal-class fairs and theme fairs. The last BIE-sanctioned universal-class fair in North America was in Montreal in 1967; last summer's world's fair in Knoxville was the most recent theme fair.

The United States hosted its first world's fair in 1854 in New York City -- an event marked by the introduction of the elevator. By the 1920s, such expositions had become so popular that France called for the formation of the BIE. However, this country didn't join until 1968, when BIE members began refusing to participate in non-sanctioned fairs.

Commerce's exposition office is responsible for both kinds of world's fairs held in this country, while the USIA handles U.S. participation in expositions overseas. Commerce invites foreign countries to participate in U.S. fairs and seeks appropriations for a U.S. exhibition hall. It also works with the fair's commissioner, a presidential appointee.

The expositions office has been criticized in the past because it didn't get BIE to sanction a universal fair marking this nation's bicentennial. More recently it was faulted for building a $20 million exhibition hall for the Knoxville fair before finding anyone to buy it after the six-month fair closed. As a result, Congress passed a law requiring Commerce to build a temporary hall if it can't find a buyer beforehand.

The difference between universal and theme fairs can be big, Morse said. Knoxville attracted 11 million visitors and cost the city and federal government less than $100 million. Commerce has projected that a universal fair in Chicago will cost the federal government $90 million, on top of the $776 million convention executives have already pledged. Nearly all of BIE's members would construct exhibition halls, Morse said, and the fair could be expected to attract at least 50 million visitors.

A final decision on Chicago's fate is expected Dec. 30, when the BIE members' votes are tallied. But first there's another matter that must be resolved.

It turns out that France wants to hold a world's fair in 1989 to commemorate the 200th anniversary of the French Revolution. Ten years ago, the BIE voted to sanction only one universal-class fair every 10 years, which means that the world could mark 1789 or 1492, but not both. France will argue that both fairs can be held because they are in different decades.