STEVENS POINT, WIS., JUNE 14 -- Until a few months ago, President Reagan seemed to be covered by a kind of blanket immunity from personal ridicule.

It didn't extend to cartoonists, satirists and others with special license to poke fun, but it was rigorously observed by politicians -- especially Democrats -- who cared about their future.

No more. Rep. Richard A. Gephardt (D-Mo.), one of the presidential hopefuls who addressed 1,000 activists attending the Wisconsin Democratic Convention here this weekend, took a shot at the president's intelligence.

Gephardt told a hoary political joke about puppies who turn Democratic the day their eyes open -- a joke Reagan has used for decades, with the parties reversed.

In Gephardt's rendition, Reagan is about to buy one such puppy, and he is "sort of shaking his head back and forth that way he does . . . {pause} . . . maybe because there's not a great deal in it."

It was an aside, unessential to the punch line, but it drew enough light laughter and applause to interrupt Gephardt for a moment before he got to the end. "The president has been unmasked, and he's fair game now," state party Chairman Suellen Albrecht said later, musing on Reagan's tumble in popularity since the Iran-contra affair.

The repercussions of that public fall were palpable this weekend -- in a state that has proclaimed sisterhood with Nicaragua and at a party convention where Reagan's contra funding, "Star Wars" proposals and domestic policies came under withering attack. They were criticized by Democratic hopefuls Gephardt, Massachusetts Gov. Michael S. Dukakis, Jesse L. Jackson and Sen. Albert Gore Jr. (Tenn.); Illinois Sen. Paul Simon's wife, Jeanne, and, on a scouting mission, the undecided Arkansas Gov. Bill Clinton.

Their tough rhetoric had the partisan blood running. Star Wars is "idiocy . . . a magic gizmo," sneered Gephardt. Reagan's Central American policy has been "a fiasco from the start" said Dukakis.

"This is the most upbeat convention we've had since 1976," said Rep. David R. Obey (D-Wis.), standing not far from a huge banner draped above the convention stage that caught the theme of the gathering: "Just Can't Wait for '88."

But for whom are they waiting? Judging from buttons and informed guesses, less than 15 percent of the state convention delegates have figured out a presidential horse to call their own -- a far cry from the convention four years ago, when Wisconsin Democrats held a presidential straw poll that forced the candidates to spend hundreds of thousands of dollars, the activists to choose sides, and the winner, Sen. Alan Cranston (D-Calif.), to peak about nine months ahead of schedule. It was a victory from which he never recovered.

This time there were no ballots and few buttons. Dukakis had the best weekend politically, lining up endorsements from former governor Tony Earl, Assembly Speaker Tom Loftus and much of the leadership of Gary Hart's 1984 and 1988 primary campaigns (Hart carried the primary here four years ago).

"This state is Dukakis' for the asking," said Loftus, thought by many here to be a future gubernatorial candidate. "He's a liberal who can show economic results, and that's a real good combination for this state."

The Massachusetts governor arrived here following a successful campaign day in Minnesota, where he drew huge crowds and was escorted around by St. Paul Mayor George Latimer, a supporter. "He's real comfortable in the upper Midwest," Loftus said.

Dukakis' speech to the Wisconsin delegates was on foreign policy. But to warm the audience, he opened with the saga of his Greek immigrant parents and closed with a paraphrase of an ancient Athenian maxim, pledging never to bring dishonor upon one's country.

He also referred -- as he does almost compulsively -- to his only political defeat, an upset for governor in 1978. Aides have told him it makes him more human; he has taken the advice to heart. In one two-minute stretch before a business audience earlier Saturday in Milwaukee, he told of having been "laid off," "involuntarily discharged," "given an unplanned sabbatical" and of discovering there is "life after death."

"I'm having the time of my life," Dukakis also told a roomful of ex-Hart supporters here, and he genuinely appeared to be.

Gephardt's convention speech had a sharper edge than Dukakis'. It was punchier and more partisan. On trade, for example, he said: "Reagan sounds like Rambo and acts like Bambi."

He, too, got good reviews. "There's an amazing improvement in him since he gave a Jefferson-Jackson Day speech here a year ago," said Albrecht. "It's quite dramatic." Gephardt said he hears compliments about improved delivery so often he's beginning to wonder how bad he used to be. "It's like taking ground balls -- you get better with practice," he said.

Jackson drew boisterous applause for his message of economic populism. Gore got off the driest line about the anonymity of the Democratic field. "Did you hear they have a sign in the Des Moines Airport?" he told the delegates. "Advance men, check your claim tags. Many candidates look alike!"

Gore's speech was full of the future. "History itself is speeding up," he said. "It almost seems as if we are moving to some kind of crescendo." He talked of the greenhouse effect, the ozone layer, genetic engineering, supercomputers and robotics.

"It's a cross between {authors John} Naisbitt and {Alvin} Toffler," said one unimpressed delegate, who asked that his named not be used. "It's future-babble." But many in the audience said they liked Gore's message and his soft preacher-like cadence. They said they had fond memories of his father -- a pro-civil rights, anti-Vietnam war senator who was defeated in 1970 after 34 years in Congress.

Jeanne Simon gave a spirited defense of her husband's old values and his non-slick appearance, and issued an unusual invitation to make a point about his life style: "The Miami Herald is welcome to stand in our garden anytime."

Clinton, the Saturday dinner speaker, dazzled his audience with a treatise on international economics, interspersed with down-home Arkansas political stories. "It was like a one-on-one conversation with 1,000 people," marveled David Ahrens, a union research director.

Clinton said he'll decide in a few weeks whether to join the presidential chase. At last sighting, he was holding court with recruits near the hotel lobby early today.