On Sunday afternoon, President Luis Herrera Campins will declare open the ninth Pan American Games, thus burying the skepticism that has surrounded the competition ever since it was awarded to Venezuela four years ago.

Some said the feuding sports and political factions within Venezuela never would reach the necessary accommodation to permit the games to proceed. At one time, U.S. officials had reserved hotel space in Puerto Rico, believing the games might be shifted there. U.S. swimming people made preliminary plans for an invitational meet in Honolulu, in case the Pan Am Games were not held at all.

Throughout the moments of distress, the Venezuelans insisted they would carry on as planned, and so they have. If a few corners have been cut to enable the competition to be held, it seems of small matter, considering the enthusiasm of the hosts.

If this is a time of pride for Venezuelans, however, it remains an occasion of mixed feelings for U.S. sports officials. It is difficult for the United States to emerge with a favorable status from the Pan Am Games.

Should the Star-Spangled Banner be heard too frequently during victory ceremonies, there will be understandable resentment of the success enjoyed by the colossus of the north. If the top U.S. athletes are kept at home to promote closer competition, there is sure to be criticism that the United States does not think much of the games.

"The Pan Am Games have more political impact than swimming impact," said Ray Essick, executive director of U.S. swimming. "If we send a poor team, we're criticized for not taking it seriously. If we send a good team, we're criticized for taking advantage of the little guy."

In 1979, in San Juan, the U.S. elected to send its top swimmers and captured all but one gold medal, Canada's Anne Gagnon prevailing in the 200-meter breaststroke.

This year, with no other major worldwide competition to distract, the U.S. has sent its best again, and only a trio of world record holders--Brazil's Ricardo Prado and Canada's Victor Davis and Alex Baumann--can realistically hope to break its grip on aquatic gold.

In track and field, because of the world championships now under way in Finland, a U.S. squad minus most national champions figures to be less dominant, although it nevertheless is likely to take a majority of the gold medals.

One of the few other sports where the leading eligible U.S. athletes are not present is gymnastics, because of the differing rules structures here and in the world championships, set for Budapest in October.

"We've sent our second team to the Pan Am Games," said Don Peteres, the U.S. women's Olympic coach. "We have trouble with the Pan Ams. They're a bummer for gymnastics. They change the rules concerning the compulsories for it and it's so close to the world championships, our athletes can't change their compulsories training. We'd like to go with our best team and win. I don't see why they can't use the world championships compulsories."

There is a different attitude in fencing, where the Pan Am Games were given priority over both the World University Games and the world championships.

"You send your best where you think they can win," said Bernie McGovern of the U.S. Fencing Association. "You need as much as six weeks to psych up and come down from a major competition, so obviously the same people can't go everywhere."

Whether even the best U.S. fencers can win here is questionable, because of the presence of the Cubans, who took six of the eight fencing gold medals in San Juan.

No country tries harder in the Pan Am Games than Cuba, which savors any victory over the United States. Besides fencing, Cuba figures to be an important factor in boxing, gymnastics, judo, rowing, weightlifting, Greco-Roman wrestling, baseball and volleyball.

Canada is a contender in basketball, swimming, gymnastics and rowing. Otherwise, only Argentina's field hockey team and the rare star from another nation, like Brazilian half-miler Joaquin Cruz, is likely to interrupt the Star-Spangled Banner.

"I've been to every Pan Am Games since (the first in) 1951 and I think this is the best team overall we've ever had," said Harold Zimman, chairman of the USOC public relations information comittee. "We don't have our best in track and field or gymnastics, but we're very strong overall."

One sensitive area will be tested early. The U.S. managed to qualify for the soccer tournament by beating Canada in Hamilton, Ontario. However, Monday's opening of actual competition will send the U.S. against defending champion Brazil and a defeat means virtual elimination, since only the champion of each of four groups will advance to the semifinals. The third team in group B is Bermuda.

If the U.S. men's basketball team is to avenge the loss to Canada in the World University Games, it must wait until the final, since the teams are in opposite groups. The women are listed for a seven-team round robin, however, with the key contest between the favored U.S. and Cuba's defending champions set for Aug. 23.

The U.S. women will play Peru Monday morning. The men will open against Mexico Monday night.

Besides soccer and basketball, competition is scheduled Monday in baseball, fencing, field hockey, judo, weightlifting, softball, tennis and shooting.

Competition in 25 sports will conclude Aug. 28. Almost 5,000 athletes, including 561 from the U.S., are scheduled to participate.

It was announced today that Ecuador will be the host for the 1987 Pan American Games, taking the place of Chile, which bowed out because of a troubled economy. The competition will be staged in Quito and Guayaquil.