This city was supposed to be the site of the first spectacular of the 1980 presidential season, a fabulous two-event affair that would set the tone for the early stages of the campaign year.

President Carter would be here, squaring off for the first time against his archrival, Sen. Edward M. Kennedy, and California Gov. Edmund G. (Jerry) Brown Jr. Six of the seven major Republican hopefuls would square off against one another. The television networks decided to broadcast it all live, 3 1/2 hours of prime time for two separate debates.

"It was a classic hype for the big show," says Republican state chairman Steven Roberts. "It was like the carnival coming to town."

But that's all changed. A week ago Carter backed out of the encounter, claiming he was too busy with events in Iran and Afghanistan. The commercial networks quickly followed, dropping plans for live coverage.

Republicans will still hold a debate Saturday night which will be broadcast live on public television and delayed on CBS. Coming two weeks before Iowa's precinct caucuses, the first test of the 1980 presidential campaign, the GOP debate could prove important.

But the Republican affair lacks the drama of a head-to-head confrontation between an incumbent president and the heir to the Kennedy family legacy.

Also, Carter's nationally televised speech tonight responding to the Afghanistan situation upstaged the Republicans on potentially their most volatile issue -- his alleged "policy of weakness" in foreign affairs. The speech reflected in its get-tough tone -- if not specific detail -- what many of the candidates have been saying privately for months.

Then, too, GOP frontrunner Ronald Reagan has decided to pass up the debate, as he has all meetings with opponents. His six challengers have met numerous times during the last year, usually agreeing on more than they disagree on.

Saturday's format, featuring four panelists asking each of the candidates the same question, also works against dramatic confrontation.

And perhaps most important, the debate will have only a fraction of the viewers it would have had had it been broadcast live during prime time on the commercial networks.

Political operatives of all candidates are downplaying the significance of the encounter. They insist their bosses will take it all in stride.

"We've never thought the debate would become a major factor in the Republican race," says Rich Bond, chief of the George Bush operation in Iowa. "It just didn't catch the imagination of people like the Kennedy-Carter matchup did. Unless someone makes a major guffaw it's a classic nonevent.

"It may hurt someone but I don't think it will help anyone very much."

With the exception of longshot candidate Rep. John Anderson (R-Ill.), who insists he is doing nothing to ready himself for the debate, each of the candidates has spent at least half a day preparing.

The big unanswered question is how far the candidates will go in attacking Carter's handling of the Iranian crisis. After a two-month moratorium on criticism of the president's policies, GOP national chairman Bill Brock opened the door for a free-swinging dialogue when he declared this week that Carter is pursuing "a policy of weakness."

Sen. Bob Dole (r-Kan.), Sen. Howard H. Baker Jr. (R-Tenn.), Bush and John Connally followed with critical statements of one sort or another.It is uncertain, however, if they will come out like lions or lambs at the debate.

"We could end up seeing a gunfight at O.K. corral on Saturday night," says GOP state chairman Roberts.

While Carter's Afghanistan speech may have stolen the punch from wholesale Republican attacks on his foreign policy, its call for a partial grain embargo opened up a weakness in farm policy that the GOP could exploit.

The other major imponderable is what the GOP hopefuls will say about Reagan. Everyone involved expects them to note the former California governors' absence, but it remains to be seen how hard they will go after him.

Reagan's forces insist they aren't worried. "As things have evolved here, interest isn't particularly high for the debate," says Peter McPherson, who is heading Reagan's Iowa operation.

Carter forces are taking a similar tack, insisting that the president's backing out of the debate has not caused an uproar in the state.

Reagan's position is somewhat different from Carter's.For a year he has steadfastly refused debates.

Carter, then trailing Kennedy badly in the polls, agreed to enter the debate Nov. 6 after James P. Gannon, executive editor of the Des Moines Register which is sponsoring the event, accused the White House of not caring about "some hick paper in Iowa."

The same hostages whom Carter cited last week as an excuse to avoid the debate were in captivity two months ago when he accepted. The big change is Carter's jump ahead of Kennedy in the polls.