THE Democratic establishment came back with a vengeance in New York's primary. Walter Mondale scored among almost every major Democratic group - labor, white ethnic groups, the big cities. He cut into Gary Hart's territory among the young affluent professionals and in the suburbs. Jesse Jackson continued to roll among blacks, the other major Democratic bloc, and showed new strength among young first-time voters.
The result: Hart got squeezed.
After Pennsylvania's primary next week, where New York's pattern could be repeated, Hart's campaign has to put into dry dock for major repairs. ''He needs the breather he's going to get after Pennsylvania,'' one of his supporters readily concedes. ''He needs to rest, regroup, rethink his personality, his persona, as well as his ideas and policies.''
The early New Hampshire boomlet for Hart, which carried over into Florida and was still running this week in Wisconsin, came so fast that people have wanted a second look.
Mondale has played the boomlet rebound tenaciously for three weeks now. In Illinois and New York he has put the contest with Hart in his own terms, stressing experience, trust, leadership. Tactically, the debates have proved a boon to Mondale. He has kept the second-look focus on Hart. The message that helped Hart at first - that people should turn from Mondale and politics of the past - lost its zip.
The Hart campaign has seen all the Yuppie stories - the infatuation of the young urban professional voters with Hart - that it cares to see. Hart is losing the compassion vote. Blacks, seniors - the vulnerable in society - are not seeing him as their candidate. This is an important impression even for middle-class Democrats and independents, for whom the tradition of caring for the vulnerable is important. Sen. Edward Kennedy carried this vote in his New York upset of Jimmy Carter in 1980. It's going to Mondale and Jackson now.
On foreign policy, Hart lost out to another grouping of Democratic voters who went for Mondale. These used to be called ''Humphrey Democrats.'' They were part of Sen. Henry (Scoop) Jackson's New York primary success in 1976.
Hart, by saying he would curtail United States commitments in Central America , pull down troop levels in Europe, and avoid US intervention in the Persian Gulf without a joint front with Europe and other oil users, left himself open to suspicion among more hawkish voters, or those who want to see more consistency in US foreign policy. Some saw such stands as inconsistent with his promises to stick by Israel.
Hart's approach left Mondale as the more centrist candidate in foreign policy , somewhere between Hart and Reagan.
In any event, Hart needs another breakthrough, to crack Mondale's steady 3 -to-2 margin in convention votes.
Jesse Jackson has his own momentum rolling among the black electorate. He must, however, quickly repudiate the threats of black Muslim leader Louis Farrakhan on the life of a Washington Post reporter, Milton Coleman, who first reported Jackson's ''Hymie'' remark. The Muslim threat could again mire his campaign in controversy.
Jackson has yet to expand his campaign to the white community, or to draw in sufficient numbers of Hispanics or others to form an effective ''rainbow coalition.'' But with the black vote alone, he already stands a change to prove a strong force at the convention.
With fewer than half the San Francisco delegates chosen, it would be foolish to think the Democratic campaign is over. Among big swing states, Mondale has won Illinois and New York, Hart has won Florida. Next month, a rested Hart will face Mondale in Texas. Then Ohio, and California.
The burden of proof in the campaign has fallen on Hart to stage a comeback.