DES MOINES -- The presidential campaign of New York Republican Jack Kemp is in early trouble. The candidacy has failed to meet the high expectations it had generated. Those expectations were encouraged in part by enthusiastic and organized floor demonstrations for Kemp at the last two party nominating conventions, where the New Yorker was a major player. At Dallas in 1984, more than one quarter of the delegates were willing to self-identify as being for Kemp in '88.

But this was about a lot more than balloons and bunting. Before Jack Kemp sold his patented solution of optimism and prosperity through tax cuts to 1980 candidate Ronald Reagan, the GOP had been mostly a grim lot prescribing hair shirts and cold showers for the country. Kemp may well have saved Ronald Reagan from the scrapheap of presidential semifinalists.

To understand Kemp's current problem, just recall the two reasons given by the 1980 Smart Money to explain why Reagan was manifestly unelectable: his conservative ideology and his advanced years. Of course, both so-called liabilities developed into political assets. To an electorate then disenchanted with the Carter administration's indecisiveness, Reagan's vaunted inflexibility became welcome constancy. In combination with his avuncular style, Reagan's age helped effectively deflect the charges of right-wing recklessness. Evidently mature arteries can cool hot blood. Reagan's age reassured voters while his moderate manner tempered his strong message.

That's not the way it's now working for candidate Kemp, whose mission seems more to redeem his listeners than to reassure them. Kemp's passion, intensity, and fervor, as observed at the Midwest Republican Conference here, are almost palpable. Together they confer a near-frantic quality on the Kemp presentation, and frantic is not presidential. Kemp's sustained high energy makes him seem younger than 51 and can leave an audience informed but exhausted.

That Kemp campaign, dissatisfied with the lack of political progress in Iowa, has seen the replacement of its two top operatives in the state. The man responsible for the replacing, Charlie Black, Kemp's respected manager, concedes the problems ''cost us three months here'' but insists he is not worried. Black says Kemp's support will come from ''the Right-to-Life groups, evangelicals, and senior citizens.''

Kemp has not advocated big domestic budget cuts, and unlike most congressional Republicans, he refuses to vote for a balanced-budget constitutional amendment. In an admirable example of intellectual candor, he admits the nation can't simultaneously have both tax cuts and a balanced budget.

Apparently, in the respected judgment of one Republican statesman, Kemp has an unspoken problem with the right wing of the GOP. The candidate, in the view of this party leader, hurts himself among a number of conservatives by ''being a 'known friendly' to blacks'' and ''sadly that's frequently not a positive credential with that constituency.'' That may be true because in a 15-minute speech to party regulars, Kemp no fewer than 10 times advocated specific overtures and initiatives to blacks and minorities.

If politics were fair, Gary Hart would not have been touted as the Ideas candidate of 1988; Jack Kemp would have. In 12 years in the Senate, half that time in the majority, Hart offered a number of thoughtful proposals. In nine House terms, all in the outgunned minority and with no seat on the Ways and Means Committee, Kemp overhauled the nation's tax code. Hart made a contribution; Kemp made history.

But that's not happening this time. Iowa is almost a pacifist state where every member of the congressional delegation voted against aid to the contras and where Kemp's advocacy of the Strategic Defense Initiative is not a galvanizing idea. No Hawkeye audiences are lifted out of their chairs by a proposed return to the gold standard. Supply-side is already on the books and remains an idea on which the jury is still out.

For Jack Kemp, his fundamental problem possibly goes back 30 years. In 1957, he graduated from Occidental College, where he majored in physical education, a fact which practically nobody else remembers and which Kemp seems unable to forget, as he awkwardly sprinkles his one short speech with ''matrix,'' ''impresario'' and ''per se.''

Ronald Reagan, because he was so unmistakably comfortable with himself, added personal serenity to the presidential characteristics we will be looking for next time. Up to now, Jack Kemp is neither serene nor reassuring.