DES MOINES, JAN. 15 -- Former Colorado senator Gary Hart rejoined the Democratic presidential ensemble here tonight. But he did not steal the show.

In his first debate appearance since getting back into the race last month, Hart said his personal sins should not disqualify him for the presidency and asked to be judged instead on the morality of his civic values.

"I am a sinner. My religion teaches me that all of us are sinners," Hart said in response to a question about adultery and judgment. "The question is whether that disqualifies us. There is another level of morality at stake here. This administration has been bankrupt in its commitment to public ethics. I would never lie to Congress. I would never shred documents. I would never sell arms to terrorists."

Hart's pledge drew a conspicuous lack of applause from the audience of several thousand at the Civic Center auditorium.

And for the remaining two hours of the televised debate, while he returned to the theme of public morality several times, he was never the dominating stage presence that some of his rivals feared he might be.

Neither were any of the others, however. This debate -- at least the 15th of the campaign -- lacked any grand argument, new material or memorable one-liners.

After the debate, Iowa Rep. David Nagle, a Democrat who is neutral in the presidential race, observed: "Six guys tied and one guy lost. The candidate of new ideas {Hart} might have shared just one of them with us tonight."

This debate may be the most important event of the Iowa campaign, watched on public television by an estimated half of the 100,000 Democrats expected to participate in the first-in-the-nation caucuses here Feb. 8.

The candidates all adopted a play-it-safe strategy, engaging occasionally in tactical thrusts and parries with a rival, but more often content to state their case and try to solidify and expand their support among activists they have been courting here for a year or more.

The debate roamed over familiar arguments about fiscal, farm and trade policy, and familiar agreement over the need to reduce aid to the contras and reach out to seek deep cuts with the Soviet Union on intercontinental ballistic missiles.

The closest thing to a Democratic theme of the evening was put forth by Rep. Richard A. Gephardt (Mo.), who took on President Reagan for for saying that "greed and selfishness and get-mine-now is the highest value in the land. We've got to get back to a sense that we care for one another."

The others voiced similar calls for caring and community. Jesse L. Jackson, long associated with such appeals for social justice, issued a different sort of moral challenge.

Asked if the country is ready for a black president, he responded, "I have watched America take tests over the years," referring to the fights for civil rights, voting rights and open public accommodations. "We will never know how America will respond to the test until it's given. I do not believe that the American people are so morally bankrupt that they cannot rise to the challenge . . . . If I am the best candidate, I expect the American people to fully accept the Americanism of Jesse Jackson."

Recalling that today was the birthdate of Martin Luther King Jr., Jackson called on voters who respect what he has done in standing with farmers facing foreclosure, citizens opposed to U.S. policy in Central America, AIDS victims and others, to support him for president. "Jesse Jackson can win. America can win."

Other highlights of the debate included a sharp exchange between former Arizona governor Bruce Babbitt and Sen. Albert Gore Jr. (Tenn.) on taxing Social Security benefits and a national sales tax; an attack by Gore on Gephardt's votes for tax breaks to wealthy corporations he now attacks, and a testy exchange in which Babbit and Massachusetts Gov. Michael S. Dukakis reacted sharply when asked by a guest Republican questioner, Illinois Gov. James R. Thompson, why they had refused to send their state's National Guard troops to training exercises in Honduras.

Hart tried to show his expertise in foreign policy, in part through questions he asked his opponents on how they might deal with Soviet leader Mikhail Gorbachev. But his rivals refused to engage him in a broad discussion of the world, preferring instead to ask Hart narrower questions on domestic programs.

Here is a summary of the points made by each of the candidates other than Hart and Jackson:Babbitt sought to portray himself as the candidate of candor and honesty.

"Don't pay attention to the polls," he said, referring to surveys that generally show him far down in the pack. "Ask who should be president. On Feb. 8, I want you to stand up for Bruce Babbitt, to stand up for America."

Responding to a question from moderator James Gannon, editor of The Des Moines Register, which sponsored the event, Babbitt called his programs of higher taxes and subjecting certain social programs to a needs test as one of political courage, not political suicide. And in a veiled reference to Hart, Babbitt said that issues of character were at the heart of his message.

Dukakis spent much of the evening ripping the Reagan administration and polishing his liberal credentials at the same time. He defended his lack of foreign policy experience with a swipe at Vice President Bush, saying the candidate for president "with the longest resume" is someone who "sat next to the president and did practically nothing" to stop arms sales "to the ayatollah" and who went to the Philippines in the early 1980s and praised former president Ferdinand Marcos for his devotion to democracy.

Asked by Thompson how he would handle nations like Spain and Greece, who want the United States to reduce its military role in Europe, the Greek-American joked, "I certainly could negotiate with Greece successfully and I speak a pretty good brand of Spanish and could do some business there." But he used Thompson's question to attack the administration's policies in the Persian Gulf and Central America.

He stressed anew his credentials as a governor who has worked to create jobs in Massachusetts and who would use the presidency to revitalize the American economy. Gephardt, who has tried to rejuvenate his campaign here with a sharp-edged populist message, spent his share of the two hours appealing to the constituencies he believes can give him a respectable enough showing on Feb. 8 to keep his candidacy alive. They are farmers, workers and the elderly. Both in his questions for the other candidates and in his answers, he targeted those voters repeatedly.

Gephardt and Gore tangled over whether Gephardt has changed his spots to win liberal votes in Iowa. Gore noted that Gephardt's new ads include "a good deal of corporation bashing" and asked how that squared with Gephardt's vote for Reagan's tax cuts, which reduced corporate taxes dramatically, and his vote against increases in the minimum wage.

Gephardt countered that he voted for the Reagan tax cuts after the Democratic alternative was defeated and then spent the next several years promoting tax reform, which resulted in 1986 in a tax bill that shifted the burden from individuals to corporations.

He said he voted against raising the minimum wage because the legislation at issue included indexation. He said he now supports a rise in the minimum wage without indexation. Gore used the nationally televised forum to keep himself before the voters in other parts of the country while avoiding too much direct criticism of Democrats in Iowa who will kick off the nominating process on Feb. 8. His opponents took note of his absence from the campaign trail here by regularly welcoming him back to Iowa. "I thought they were going to put your face on milk cartons," Babbitt quipped to Gore.

"It's no secret," Gore told the audience, "that I'm taking an approach to the role played by the Iowa caucuses that is very different than my colleagues. I ask for your help in carrying Iowa in the general election."

Paul Simon, who is the leader in most polls of likely caucus attendees, used tonight's debate to try to nail down his role as defender of traditional Democratic Party liberalism. Invoking the name of Hubert H. Humphrey, Simon said he was committed to the values Humphrey fought for during his polical career.

"Hubert had dreams and he acted," Simon said. "He didn't get every dream . . . but we're a richer, finer nation" because of what he did. "If you want a president committed to that traditon, I stand ready."

Simon's one moment on the defensive came when Babbitt asked him whether he would quit taking money from political action committees in his presidential campaign. Simon said he favored public financing and limits on how much money can be spent in political campaigns, but added he is not ready to quit taking political action committee (PAC) money.

"I'm going to use the system as it exists and I'm going to be a leader in changing it," he said.