The 1988 presidential primaries are over, but Michael Dukakis and George Bush are still campaigning, now seeking party unity at the summertime conventions in Atlanta and New Orleans and into the fall.

Each celebrated victorious conclusions to the long primary season Wednesday, with Dukakis declaring, "The marathon is over and now the race to the finish line begins."Both men, assured of their parties' presidential nominations, pledged a tough but clean election fight offering voters a clear choice.

The magic number for a convention majority was 2,081, and Dukakis easily surpassed that with his landslide finale. His delegate total after Tuesday's sweep stood at 2,251, according to the latest Associated Press count.

Confronted by polls saying Dukakis was the current choice of a majority of voters, Bush said, "I'm fighting back. I'm the underdog now."

Both nominees-to-be are reaching out in the next few days to the men they defeated in the 39 primaries, seeking to heal any wounds and to avoid any new squabbles. And part of that process will be their choice of vice presidential running mates for the general election.

Bush joins four of his five former Republican rivals for a "Unity '88" meeting in Denver on Friday, the first of several such meetings to lay the groundwork for the GOP National Convention in New Orleans Aug. 15-18.

Dukakis, his final primary victories in hand, gathers the formal backing of some former Democratic rivals Wednesday. He will stop in St. Louis on the way home from California to receive the endorsement of Rep. Richard Gephardt, while Sen. Paul Simon of Illinois will add his support in a statement.

"I don't see any great divisions within our party," Dukakis said before the votes were counted Tuesday. "We're going to have ourselves a great convention."

For the Massachusetts governor, the first challenge is Jesse Jackson.

The preacher-turned-politician has been Dukakis' most persistent opponent, hanging on to the end of the primaries and promising to continue all the way to the Democratic National Convention in Atlanta July 18-21.

"Suffice it to say, we're going to keep our campaign alive to July, at the convention," Jackson said in Los Angeles.

In the final contests, Dukakis defeated Jackson by margins of better than 2 to 1 in New Jersey, New Mexico and Montana. Returns from California were putting him ahead in that state by a comparable margin.

Jackson has stepped up his pressure on the Massachusetts governor in the closing hours of the season - on the vice presidency and on the issues. In his strongest statement yet, Jackson suggested he had earned at least an offer of the No. 2 spot on the ticket.

"If he were to win, extending the invitation to me is his option," Jackson said of Dukakis. "It is an option my constituency has earned."

Dukakis brushed aside such comments, noting that Jackson has said that it is up to the nominee to select a running mate. And he also claimed to be unconcerned about the possibility of fights at the convention on issues Jackson thinks are important.

"We may have a floor fight or two," Dukakis said. "There may be some issues that go to the floor."

But Jackson was conciliatory as well after a meeting Monday night with Dukakis.

"He's sensitive on these matters, and we're going to have some follow-up meetings, necessarily to discuss matters of platform, credentials, rules, the convention itself, and beyond that, our strategy for winning in November," Jackson said.

Jackson aides said he would remain in California for a week of rest. He will continue to campaign through the convention.For his part, Dukakis will campaign remarkably hard for the next six weeks.