BOONE, IOWA, FEB. 4 -- Rep. Richard A. Gephardt (D-Mo.) returned to Iowa today and studiously avoided the sniping by his main rivals in the Democratic presidential race.

Gephardt, considered a narrow front-runner going into Monday's Iowa precinct caucuses, found himself the target of a radio advertisement by Sen. Paul Simon (D-Ill.) questioning his votes in favor of the Reagan tax cuts, various weapons systems and a Social Security cost-of-living freeze.

At a breakfast in Des Moines, Simon singled out Gephardt's 1981 vote for President Reagan's tax cut, which he said caused the sizable budget and trade deficits, and noted that Gephardt now rails against these deficits in his tough talk on trade.

Asked about Simon's ad, Gephardt said in Ames, "It's disappointing. Sen. Simon said he would not engage in negative advertising, and I'm sorry to see he is." But he added, "He's a friend, and he'll be a friend after this campaign is over."

Gephardt, who had told Iowa supporters that he would "lead the fight" in the House Wednesday against President Reagan's request for $36.2 million in aid for the Nicaraguan rebels, returned today proclaiming that "we beat the president."

But he said little about his role in the House's rejection of the administration request. Instead, he repeated his main campaign themes on trade and farm policy, care of the elderly and education.

But Massachusetts Gov. Michael S. Dukakis, who with Simon is challenging Gephardt for the lead here, took a veiled swipe at Gephardt's leadership on the contra-aid issue.

At stops in Davenport and Cedar Rapids, Dukakis pointedly did not mention Gephardt's role in the debate but asked listeners to "drop a note to Speaker {Jim} Wright and thank him for his leadership in finally ending the fiasco of American aid to the contras."

Asked at a Des Moines news conference about Gephardt's role, Dukakis said, "I'm grateful to him for his vote, and I'm grateful to Speaker Wright for his leadership."

Meanwhile, four of Gephardt's leading supporters in Iowa -- state House Speaker Don Avenson, state Senate Majority Leader Bill Hutchins, Lt. Gov. JoAnne Zimmerman and State Treasurer Mike Fitzgerald -- told reporters that they "deplore" what they described as the first use of a negative campaign commercial in the history of the Democratic presidential caucuses.

Memories appear to be short. In 1980, then-President Jimmy Carter sharply attacked Sen. Edward M. Kennedy (D-Mass.) in his ads during the caucus campaign here.

"Iowans don't like negative campaigns," Avenson said, predicting that the "desperate" tactic would backfire.

The Simon campaign countered with former state party chairman A. Arthur Davis, who responded that the four had "simply mistaken what a negative commercial is. It has always meant an attack on character. All we do is talk about his record. It reminds me of what Truman used to say about the Republicans: 'I don't give 'em hell. I just tell the truth, and they think it's hell.' "

The ad says Gephardt voted for the 1980 grain embargo against the Soviet Union, a 1985 amendment to wipe out cost-of-living increases for Social Security recipients, the MX missile and B1 bomber, chemical weapons, the neutron bomb and Reagan's 1981 tax cut. Simon voted against all of them.

"All we did was compare Simon and Gephardt's voting record," said Simon media adviser David Axelrod. "If we wanted to get negative, we could have compared Gephardt and Gephardt's voting record," a reference to the fact that Gephardt has changed his position over the years on several key issues, including the MX and abortion.

A Gephardt spokesman said that, while the ad

correctly cited his votes, they were taken out of context.

Dukakis, campaigning in five cities, told reporters that he plans to "stay positive and upbeat" in his closing speeches and advertisements.

In speeches, he did not mention his rivals, but he told high school questioners in Cedar Rapids that he had policy differences with Simon and Gephardt.

"Paul and I share the same goal of full employment," he said of Simon, "but the way to do it is not with a glorified WPA," referring to the Works Progress Administration, the New Deal jobs program, and Simon's suggestion that the government hire the long-term unemployed rather than pay unemployment benefits.

Answering a question about Gephardt's trade proposals, Dukakis said Gephardt was brushing over "deep-seated problems."

"We have a lot of tariffs and quotas ourselves. The way to get rid of them is through negotiation . . . and the president has all the authority he needs," Dukakis said.

Meanwhile, Jesse L. Jackson was preparing to launch his first television ads of the campaign, featuring television's Bill Cosby. And former Colorado senator Gary Hart went on television Wednesday night to say he is not a politician but a "reformer" ready to shake up the Washington establishment.

Staff writers David S. Broder and Paul Taylor contributed to this report.