The Democratic National Committee took a major step away from the participatory politics of the 1970s today when it voted to shrink the size of the 1982 mid-term party convention almost by half and allocate most of the delegate seats to elected and party officials.

Overturning a resolution of the 1980 national convention that called for at least two-thirds of the mid-term conference delegates to be elected by grass-roots Democrats at the local level, the national committee said its own members and an equal number selected by state party committees were the people who ought to set the party's mid-term election course.

The conference will be held next spring or summer, with Philadelphia favored over San Antonio in the bidding to host the event.

The decision was shouted through by an overwhelming voice vote. Opponents said they would challenge the action before the party's judicial council and perhaps take their case to court, but acknowledged they have little chance of success in either forum.

Party Chairman Charles T. Mannat, who strongly supported the change, told a news conference that Democrats were "not in any way turning their backs on the participatory process." But the action was seen by many others as a landmark in the changing attitudes and procedures of the Democratic Party.

The notion of having a mid-term conference of grass-roots Democrats was born in the 1972 convention that nominated George McGovern -- one of many actions taken during that period to "open up" the party to women, minorities, youths and others who were described as victims of past exclusion. Such conferences were held in 1974 and 1978 and the third was ordered for 1982.

But the conferences have been viewed with suspicion by many elected Democrats, who see them as undisciplined and expensive distractions from the basic political work of the party.

In 1978, the 1,633 delegates who met in Memphis rebelled against the budget-cutting policies of the Carter administration. President Carter was forced to mount a major floor effort by virtually his entire senior White House staff and Cabinet to avoid a repudiation of his policies by the delegates, many of them supporters of Sen. Edward M. Kennedy (D-Mass.).

There were fears among many party officials that the 1982 mid-term conference would also be dominated by liberal activists and would draw off money and energy from the congressional campaign and become, as one official said, "the first primary between Kennedy and [former vice president Walter F.] Mondale."

Instead of the December, 1982, conference with at least one day of debate and voting on policy resolutions that was mandated by the 1980 convention, the action today will produce a spring or summer conference that will be used partly for political training workshops and partly for policy discussions. There is no guarantee there will be any policy votes.

There will be about 900 delegates -- 369 of them the Democratic National Committee members, an equal number to be chosen by state party committees, and 100 to be named by Manatt with the approval of the party executive committee. All Democratic governors, 24 House members and eight senators will also be invited.

Manatt said participation by women and minorities will be protected and pointed out that states will still have the option of picking delegates by local election, if they choose.

But no one would disguise the switch in direction from the 1970s. Rep. Barbara A. Mikulski (D-Md.), the chairman of one of the earlier party-reform commissions, said, "participatory democracy is a victim of inflation," adding that "we're a political party, not a think-tank. We have to concentrate on issues that help us win votes."

Donald Fowler, a South Carolina national committeeman who served on almost all the party reform commissions of the 1970s, said, "the 1978 conference was an assembly of the rabble. Next time, we'll have an over-represenation of party officials and elected officials, but that is fine with me. I think they can represent all the interests and consituencies of the Democratic Party."

Billie Carr of Texas, a veteran reformer who organized the losing fight against the change, said the conference as now designed will be "no more than an enlarged meeting of the Democratic National Committee. . . . You're going to have people outside demonstrating because they can't get in."

But, sobered by the 1980 defeat and the threat of a Republican takeover of the House in 1982, the national committee was in no mood to heed such threats.

Despite the presence of a large contingent of former Mondale aides and some Kennedy operatives, there was little talk here of the 1984 nomination. A poll of national committee members by CBS News found about one-fifth volunteered support for Mondale and one-tenth for Kennedy, with most expressing no preference.

By about the same ratio, they named Mondale over Kennedy as the person they would most like to have campaign for Democrats in their states. Only one of the 324 committee members interviewed mentioned Jimmy Carter as a helpful campaigner in 1982. He was from Guam.