BOSTON, OCT. 1 -- Massachusetts Gov. Michael S. Dukakis (D) returned to the presidential campaign trail today, saying that he has learned "a very hard and very tragic lesson" from the negative tactics that cost him the services of his two top political aides.

After visiting staff members shaken by the sudden resignations Wednesday of campaign manager John Sasso and political director Paul Tully, Dukakis made a quick swing to Hartford, Conn., with mixed results.

He picked up endorsements from the speaker of the state's House, the president of its Senate and 35 other legislators, but at the half-hour news conference designed to dramatize his support, all but one question focused on the fallout from Wednesday's disclosure that Sasso was responsible for distributing an "attack video" that undermined the candidacy of Sen. Joseph R. Biden Jr. (D-Del.), who pulled out of the presidential race last week.

Later tonight, Dukakis flew to Iowa for a weekend of campaigning in a state that will give a clearer indication of whether the tapes incident is just "a bump in the road," as one of his Connecticut backers called it, or a far more serious setback to a campaign that has moved Dukakis into second place in the polls. He trails only Jesse L. Jackson among Democrats.

His first stop in Iowa is Sioux City, where Biden's candidacy had been particularly strong and where anti-Dukakis sentiment is running extremely high today. "He says he wants to face the music. And in my opinion the music is taps," said Tim Bottaro, Sioux City attorney and state central committee member who had supported Biden. A string of Democrats predicted a sour reception in Iowa for the Massachusetts governor.

Before leaving Boston, Dukakis disclosed that a third senior campaign official, field director Jack Corrigan, had told him this morning that he, too, had a hand in distributing the tapes -- prepared by a campaign intern at Sasso's direction -- showing how Biden had lifted his closing comments at an August candidates' debate in Iowa from a campaign speech of British Labor Party leader Neil Kinnock.

Corrigan said that, at Sasso's instruction, he sent the tape to NBC News after Sasso had sent identical tapes to The New York Times and the Des Moines Register. Dukakis said he decided that Corrigan's "mistake does not merit dismissal from the campaign."

The candidate today defended his handling of the incident -- particularly his waiting from 4 p.m. Tuesday, when he said he learned from a contrite Sasso of his campaign's involvement in Biden's downfall, until the following afternoon to sever his campaign manager from his duties.

Dukakis said at a news conference Wednesday morning, where he disclosed Sasso's role, that he had rejected Sasso's offer to resign and instead had offered him a two-week leave. Afterward, with many inside and outside the campaign questioning Dukakis' decision, Sasso renewed his request to resign and announced at midafternoon that this time, Dukakis had accepted. Sasso told him that Tully also was quitting because he had lied to Time magazine in denying the campaign's involvement in the incident, when he knew Sasso had ordered the tapes' distribution.

Dukakis told reporters here today that "all this took less than one day . . . . It was rapid." Noting his "very close personal bond" to Sasso, the manager of his two most recent gubernatorial campaigns and top aide in the governor's office, he said, "You don't make decisions like this in a half-hour."

He said his first decision to put Sasso on leave represented a desire "to be sensitive to him and his family" and a wish for "him to have an opportunity to think about this as well."

Asked whether he had done all he could in the previous 19 days to assure himself that his was not the unnamed "rival campaign" that the Des Moines Register said on Sept. 12 had supplied the "attack tape" on Biden, Dukakis said, "These reports of leaks. . . are in the papers all the time. As a candidate, I do not pay much attention to them."

But today, the campaign was busy trying to quash rumors of others' involvement in the negative campaigning, including an allegation that Kitty Dukakis, the candidate's wife, had seen the tapes. Campaigning in Manchester, N.H., she said categorically, "I never knew about them until Tuesday night," when her husband told her of Sasso's confession.

At a visit to his headquarters, Dukakis said the "very hard and very tragic lesson" of the tapes was that "negative campaign efforts to hurt other candidates should have no part in my campaign {or in} American politics." Those who heard him quoted Dukakis as saying that "my heart is broken because of John {Sasso}," but warning them that he would not condone "end runs or behind-the-scenes" attacks on other candidates.

Dukakis associates, led by campaign chairman Paul Brountas, a Boston lawyer, began discussions of replacements for Sasso and Tully, hoping to fill the top slots within two weeks.

Among those reportedly under consideration are Ira Jackson, who recently left Dukakis' Cabinet to become a Boston bank vice president; J. Joseph Grandmaison, the manager of Dukakis' first campaign for governor, a veteran of several presidential campaign staffs and now New Hampshire Democratic chairman; Thomas F. Donilon, a Washington lawyer who was a key figure in the Jimmy Carter and Walter F. Mondale campaigns and a senior adviser to Biden; Kirk O'Donnell, the top aide to former House speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), and now head of a Washington think-tank; Boston campaign consultant Tom Kiley, and Anthony Podesta, former executive director of the lobbying group People for the American Way.

Some confirmed they had been sounded out, but there was no indication any of them had been formally recruited. And several indicated they are unavailable.

Meantime, early assessments of the damage to Dukakis varied widely. In New Hampshire, where polls consistently have shown Dukakis well out front for the nation's first primary, Mike Muir, director of former Arizona governor Bruce Babbitt's state campaign, said, "I don't think it's causing any defections" from Dukakis. "He's not going to be damaged by what happened." But the state campaign manager for another rival, requesting anonymity, said, "It will slow him up considerably and probably shut the door on his picking up any people from Biden," who had a strong cadre of Democratic activists.

Dukakis apologized by phone to Biden Wednesday, but sources on both sides indicated the senator's response was chilly. "I'm sure he was angry," Dukakis told reporters, adding that the reaction was justified.

Boston newspapers editorially criticized the governor today. The Globe said Dukakis' initial inclination to put Sasso on leave "was a no-decision decision that would be unacceptable in the White House" and his later decision to accept the resignation "narrowly averted a campaign disaster." The Herald said, "He has lost the moral high ground in his quest for the presidency."

In Hartford, toward the end of the rough news conference, Dukakis was asked whether he thought the worst was behind him. "I can't make that judgment," he said.

Staff writers Thomas B. Edsall in New Hampshire and Bill Peterson in Iowa and special correspondent Michael Rezendes in Hartford contributed to this report.