The returns from last weekend's Democratic caucus in Wisconsin told the story about peaceniks and their priorities. While the supporters of Walter F. Mondale drifted off to pleasanter pursuits in the sunny afternoon, the followers of "peace first" candidate Alan Cranston hung in to vote for their cause--and to lame the front-runner.

The peaceniks showed their stuff again on Tuesday at an MX rally, organized by Rep. Edward J. Markey (D-Mass.). Several hundred of them stood for three hours in brain-baking heat. They heard a score of congressmen inveigh against the arms race and cry shame on Democrats who had voted for the MX.

Three Democratic presidential contenders, Sens. Cranston, Gary Hart and Ernest F. (Fritz) Hollings, were up there pounding away at the folly of it all. Mondale did not attend. Although in the wake of the Wisconsin upset he had called the nuclear freeze "our first priority," he stayed in his office for a post-mortem sandwich lunch with his staff.

Apparently, he had thought long and hard about going. Many telephone calls were made. But in the end, he decided against it.

To some of his shaken supporters, who regarded Wisconsin as a disaster waiting to happen to his themeless campaign, an appearance at the rally would have been a way to recoup and to declare that he had seen the light.

But Mondale, whose infrastructure is better than his instincts, is more at ease talking about education, the non-radioactive issue that Ronald Reagan is making trendy. Mondale discussed schooling at a news conference yesterday with great force and fire.

The process of educating Democrats, even after a lesson that was as subtle as a blow upside the head, is hard going. Just before the rally, at a House caucus forced by incensed freshmen, the junior members of the party sought to inform their elders that they had booted the arms control issue.

Rep. Jim Bates (D-Calif.) gathered 103 signatures of support for an extraordinary, closed-door meeting, the purpose of which was to make further votes on the MX a subject of party discipline and loyalty. MX won a critical test in the House on May 24, with the help of 91 Democrats.

Except for Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), the entire leadership--James C. Wright Jr. (Tex.), Thomas S. Foley (Wash.), Gillis W. Long (La.), William V. (Bill) Alexander Jr. (Ark.), Richard A. Gephardt (Mo.)--all went along with Reagan's contention that the road to arms control is paved with nuclear weapons.

The meeting was, considering the emotion that generated it, fairly amicable. No decision was made to make the MX a party matter. But Rep. Tony Coelho (Calif.), chairman of the Democratic Congressional Campaign Committee, called it "a sensitivity session," which was "very useful."

Rep. Barney Frank (D-Mass.) confronted the architects of the Democratic "compromise"--Reps. Les Aspin (Wis.), Albert Gore Jr. (Tenn.) and Norman D. Dicks (Wash.), now derisively known among dissenting colleagues as "The Three Magi"--and told them that they had gotten "the minimum" in their exhilarating negotiations with the president. Since "minimum" was Aspin's own definition of the latest offer, it could not be disputed. Frank said that because of their investment in the deal, the three would be bound to see "flexibility" where none exists.

Markey accused the White House negotiators of the sin of "insiderism"--that particular Washington ailment that makes office-holders think their constituents appreciate more than anything else the thought that their representatives are members of the inner circle.

Tom Foley, the big, affable Democratic whip, who lives his life in the shadow of Washington state's idolized hawk, Sen. Henry M. Jackson, said that arms control is "a moral issue," which, like abortion, evokes deep feelings on both sides, and is therefore inappropriate for consideration as a party matter.

Foley was among those who attended a stormy session on the MX last week in Coelho's office. Coelho, who feels keenly that the Democrats should approach the next election armed with the arms-control edge, summoned two political consultants, Charles Ferris and Peter D. Hart, to counsel the members of the leadership.

Ferris said bluntly that his soundings outside Washington showed that the heavy Democratic vote for MX was a mistake. Foley rejoined with some heat that he was "sick and tired of hearing people say that a vote for one weapons system meant an abandonment of the peace issue."

Some Democrats believe that their erring colleagues learned something from Wisconsin and the caucus and may recant when the time comes to vote on the authorization and appropriations for the MX. Who knows, they may even have begun to understand that the biggest, deadliest, most destabilizing weapon yet invented is a front-runner in the arms race.