When he first came here a month ago, just after he had entered the presidential race, the candidate was "Ted the Invincible," a free-swinging frontrunner who talked about his political prospects with an easy confidence.

But when Edward M. Kennedy came back to this bustling riverfront city this weekend, "Ted the Uncertain" seemed a more appropiate label for a candidate who is weary of reading his own bad press notices and evidently worried about the outcome of his clash with Jimmy Carter in their debate and in the Democratic caucuses here next month.

The striking development in the first month of Kennedy's race for the White House has been the rapid demise -- both inside and outside the campaign staff -- of the once widely held notion that Kennedy, heir to the nation's richest political legend, is unbeatable.

Almost overnight, the perceived Superman of American politics has turned into just another Clark Kent. What happened? Iran had a lot to do with it, but another factor was the candidate's failure to meet the standard of performance America expects from a Kennedy.

Compared to the dozen or so other Clark Kents who are pursuing the presidency this year -- when I have seen them, at least Kennedy stacks up fairly well.

He clearly has a broad knowledge of foreign and domestic issues, he has definite (generally liberal) positions on many of them and he is reasonably consistent, if not always articulate, in stating his positions. In the past week, he was asked about illegal aliens by a Hispanic activist in Los Angeles and by a businessman in Des Moines, and gave the same answer both times. Not every politician could make such a claim.

But other politicians are not the measure by which a Kennedy campaign is judged. The standard Ted Kennedy has to live up to is the combination of brains and dynamism that Democrats think of when they reflect on John and Robert Kennedy.

And that is a measure, judging from the comments of his audiences, that Edward Kennedy often fails to meet.

"I guess Bobby never gave him no speaking lessons," a union man in Des Moines said the other night after hearing Kennedy offer verbose, tangled and ocasionally incoherent answers to voter's questions.

"I thought what he said about Iran was sort of injudicious," said a woman who came out to see the candidate in Alabama. "I thought a Kennedy would know better."

He can hardly complain about being held to this standard, because he brought it on himself. Although he promised it at the start of the campaign to eschew nostalgia, he has invoked it regularly ever since.

Every campaign speech these days contains a reference to "President Kennedy"; "my brother Bob" comes up in every second or third appearance.

Wherever he goes Kennedy is accompained by a designated sister and an assortment of his children, nieces and nephews, whom he introduces to cheers and squeals from every audience.

On his last visit here, the candidate even brought along his 89-year old mother, Rose. The spunky matriach brought the house down with an appeal that only a mother could get away with: "I know you assisted my other sons when they aspired to the presidency, and I want to thank you for giving your help to my last child."

"Being a Kennedy has obviously been advantageous," the senator said in an interview. "And I think it has been, probably, from a personal view, disadvantageous. The disadvantage is the constant measurement to my brothers' very high standards.

'And I'm realistic about that. I've understood that since I entered public life."

As he has shown on occasion, the youngest Kennedy can live up to the family standard. But on this campaign, it has been a sometime thing.

Three weeks ago, talking about farm policy in Grinnell, Kennedy gave an absolute humdinger of a speech, an articulate, impassioned blast at Carter that had the young audience cheering for more.

But he rarely matched that performance afterward. Back in Iowa a week later he gave essentially the same speech, but somehow turned it into a moribund presentation that barely drew polite applause.

Kennedy's spottiness plays havoc with the pundits, particularly the traveling political drama critics who drop in now and then to see how he's doing.

On Nov. 28, three political analysts -- one each from The Washington Post, The Wall Street Journal, and The New York Times -- spent the day with the candidate. Each interviewed the senator and followed him on the campaign trail.

The Post columnist then wrote that Kennedy appears to be waging a joyless campaign, holding back because his heart is not in the effort. The Journal's report about the same day warned that Kennedy was attacking Carter with such "zestful confidence" that he could destroy the Democratic Party. The Times' man took a middle course, saying Kennedy is "at ease on the stump" and that his campaign had "found its footing."

All three accounts were accurate, depending on which part of the day the reporter chose to focus on.

Members of Kennedy's traveling staff point out that their man's erratic performance, as contrasted, say, to the mechanical regularity of Ronald Regan or the muted steadiness of Jimmy Carter, proves that he is the most "interesting" candidate. "Interesting" he is -- as he proved last week when he turned an interviewer's routine question about the shah of Iran into a big political flap.

This week Kennedy dropped Iran from his stump speech, concentrating instead on inflation, oil prices, interest rates, nuclear power and other areas where he can poke at Carter without hindrance.

But when asked, he insisted tenaciously that he was right to raise questions now about American policy on the shah's future.

He was strengthened in that view Friday by lead editorials in the Des Moines Register and the Lincoln, Neb., Star, both of which supported his right to debate the issue.

"The media are trying to manipulate those remarks into a major political disaster," the Nebraska paper said. "We'd be surprised if that's the way the issue is perceived among the grass roots."

Kennedy volunteers in Iowa said the candidate's eruption on the shah may have hampered their efforts at least temporarily. Still, the organizers turned out large, receptive crowds to greet the senator at eight cities around the state this weekend.

Although Kennedy has spent much more time in Iowa than anywhere else, he still sounds like a regional candidate. He talks and jokes about New England wherever he goes.

"You in Iowa may be number one in corn and hogs, but I come from and agricultural state, too," he likes to say. "massachusetts is number one in cranberries."

More important, his view of the issues is shaped by geography. When he gets going -- often with great passion -- on the inflationary impact of oil price decontrol, it is the New England experience from which he draws his examples.

Sometimes, when he is really worked up about the cost of fuel oil or some other pet subject, he starts shouting about "the people of Massachusetts and New Hampshire" and neglects to include the people of the state he happens to be visiting.

But then, Massachusetts is all part of the Kennedy legend -- and the family heritage still seems to be a potent political plus.

When Kennedy gave a quiet, flat campaign speech in Hartfelle, Ala., last Thursday most of the press coprs agreed that he had done poorly. But the audience seemed to disagree.

"He seemed so dynamic," Barbara Peek a local school teacher, enthused. "But I guess any Kennedy would be that way." CAPTION: Picture, SEN. EDWARD M. KENNEDY . . . Strategists foresee a lengthy war of attrition; Picture 2, In campaign speeches, he is at times hard-hitting, other times lackluster. AP