SIOUX CITY, IOWA -- Only the offstage phantom of Gary Hart disturbed the conformity among Democratic presidential candidates in their debate here last Monday night, which did nothing to resolve the uncertainties of Iowa caucus voters less than a month before decision day.

Insiders cautiously assess the battle for first place as a struggle between Gov. Michael Dukakis and Sen. Paul Simon, but their contest stems from no disagreement on basic principles. Nor is there real dissent on issues from the other two candidates present here -- Richard Gephardt and Bruce Babbitt -- who have become Iowa's serious longer shots.

Over the past six months, contestants here have grown closer on the issues and -- with the exception of Babbitt -- less apt to risk the censure of harmony-loving Iowans by provoking each other. All advocate bigger government, trade protection and higher taxes if necessary to balance the budget.

Not once during two hours broadcast over local public television did the debaters mention the Soviet Union, for good or ill. Sen. Albert Gore Jr. might have. But having abandoned an Iowa campaign, Gore cancelled here at the last minute. Jesse Jackson, who always spices up a debate, had a scheduling conflict.

But Hart's absence was most noted, not for any special treatment of issues by him but because of the fear and loathing he excites in Iowa Democratic activists. They loathe Hart's lead in state polls and fear that he passed up the local debate here to make a splash at tonight's nationally televised Des Moines Register debate. Their apprehension was increased when Gephardt's agents let it be known backstage here that weekend trackings showed Hart's favorability rising above 50 percent in Iowa. That only intensifies anger about Hart's reentry.

In Davenport, Scott County Chairman Bob Schloemer told us he endorsed Dukakis because he felt the need to support somebody against the resurrected Hart. In declaring virtually no Hart support among probable caucus-goers in Davenport, Schloemer echoed what we heard in Sioux City, Waterloo and Des Moines. The consensus: Hart needs a miracle to match his Iowa showing of 16 percent in 1984.

Still, the phantom is feared because nobody else has taken command. Dukakis has the best organization, and Simon leads the polls of probable caucus-goers. But traveling with both, we found they light few fires.

On the night after the Sioux City debate, Simon traveled through bitter cold to speak to some 200 voters who waited two hours in a freezing fertilizer warehouse to hear him speak. Yet, talking to them after his speech, it was clear many were just shopping and remain undecided.

Their indecision may stem from nearly identical issue positions (aside from Babbitt's politically suicidal call for a regressive national sales tax). Last summer's promise of heated, perhaps illuminating, debate has faded.

Although Gephardt has returned to double-digit status in Iowa polls by falling back on protectionism, his opponents believe his new television commercials threatening South Korea with reprisals exert only limited voter appeal. Nevertheless, Dukakis does not attack him as he did last summer and, along with Simon, engages in discreet Korea-baiting.

Gephardt no longer suggests that Dukakis is a New England provincial ill-equipped for international issues, and the governor has stopped bragging about how he saved Massachusetts. Warned by advisers that his tough-guy performance in last month's NBC debate did him no good, Gephardt went out of his way here to reassure Babbitt that ''I wasn't criticizing you, only your policies.''

Having faced each other so often since they began in Houston last June, the candidates scarcely prepare for minor league debates such as Sioux City and responded to questions here with undigested hunks of set speeches. That explains why none has captured Iowa's Democrats, and why Gary Hart's phantom candidacy still stirs distress while going nowhere.