The biggest Democratic gathering in two years will be held this weekend in Memphis -- and no one, including the sponsors, is sure why they are bothering.

President Carter and Vice President Mondale will lead a contingent of several hundred top administration officials to the 1978 Democratic National Conference, where as many as 5,000 delegates and spectators are expected.

The mini-convention is being held to carry out a mandate of the 1976 Democratic National Convention, and those who show up will be asked to help pay for its $650,000 cost.

But, except for the administration officials, who are being sent to Memphis at their own expense, most of the big names of the Democratic Party will be notable by their absence at the three-day meeting.

"Our turndown list reads like a Who's Who of American politics," said Elaine Kamarck, the Democratic National Committee staff member with the thankless task of filling the 24 issues panels that will run all day Saturday.

The original idea of the liberal reformers who mandated the mini-convention was that it would provide a forum where spokesmen for the party's varying philosophies could examine -- and perhaps challenge -- the policies of a Democratic president.

But except for a panel on national health insurance -- where Sen. Edward M. Kennedy (D-Mass.) and United Auto Workers President Douglas Fraser will debate against presidential assistant Stuart Eizenstat and Secretary of Health, Education and Welfare Joseph A. Califano Jr. -- there is scant promise of fireworks.

For a time, the White House acted scared that disgruntled party activists might dump their disagreements on the president. But the post-Camp David rise in his popularity and the modest Democratic losses in last month's elections have quashed any danger of serious dissent.

The convention program constricts all formal voting on resolutions into a five-hour period Sunday afternoon, when many delegates will be leaving for home. Most of the 20-odd resolutions expected to be considered are bland or supportive of Carter policies.

The elements of the party supposedly most unhappy with Carter -- liberals, blacks, orgnized labor and farmers -- will be represented in Memphis, but it's unclear in what force.

A southern-based community-organization coalition called ACORN will probably make the most noise outside the convention with a demonstration against administration policy toward the poor. A caravan of tractor-riding farmers from the American Agricultural Movement is expected, as are Iranian students protesting U.S. support of the shah.

Inside the convention hall, two liberal groups, the Democratic Conference and the Democratic Agenda, pose potential threats to party harmony. The Democratic Conference, led by Rep. Donald M. Fraser (D-Minn.), mounted a spirited challenge to convention rules recently, but party chairman John White at least temporarily pacified Fraser with a promise of some minor concessions to give more time to floor debate.

The Democratic Agenda is an issues-oriented group led by socialist Michael Harrington, best known as author of "The Other America."

The group has collected hundreds of signatures on petitions to bring a series of controversial issues to the convention floor. At least three of them -- challenging administration policy on national health insurance, energy and inflation -- should result in floor fights.

The group also has organized liberal anti-Carter opposition in the Saturday issues workshops. "I think they [the administration] will find those workshops a lot more lively than they expect," warns Marjorie Phyfe, an agenda spokesman.

But the likelihood of any divisive debate was reduced either by the selectivity of the invitations to Saturday's issues panels or the turndowns by those who might have made headlines with their dissent.

Convention officials noted, for example, that:

On the panel where national security affairs adviser Zbigniew Brzezinski is expected to discuss the strategic arms talks with the Soviet Union, such critics as Sens. Henry M. Jackson (D-Wash.) and Sam Nunn (D-Ga.) will not attend.

On the panels where administration officials will discuss other aspects of foreign policy, Sens. Frank Church (D-Idaho), George McGovern (D-S.D.) and Richard Stone (D-Fla.), all members of the Foreign Relations Committee, declined to attend.

California Gov. Edmund G. Brown Jr. (D) said no to appearing with administration officials on urban problems.

Ditto with such independent authorities as Sens. Daniel Patrick Moynihan (D-N.Y.) on welfare, Russell B. Long (D-La.) on taxes and Edmund S. Muskie (D-Maine) on the budget.

The result is that most of the administration bigwigs should have the floor to themselves, unless a grassroots delegate decides to voice a dissent.

"Basically, it's been a really dismal process," Kamarck said. "We had a hard time getting people to run for delegate. And we've had an even harder time getting people to come for our panels."

But while expressing disappointment at the turndowns, convention officials also conceded that some of Carter's sharpest intraparty critics had been deliberately omitted from the invitation list.

William Winpisinger, president of the machinists union and head of an energy coalition involving other union and liberal groups, was not invited to be on the energy panel. Winpisinger has said publicly that the Carter energy policy is so bad for consumers that he will not support the president again in 1980.

Spokesmen for most of the big-name absentees pleaded other engagements, but an aide to Rep. Morris K. Udall (D-Ariz.), chairman of the house Interior Committee, said Udall had turned down an appearance on the natural resources and environment panel because, "given the rules and format, he decided there really won't be much to do there."

"These things," the aide said, referring to the mini-convention, "are probably best suited for out-of-power parties."

That is how they began. In 1972, the Democratic convention that nominated McGovern for president also ordered that there be a mid-term conference to adopt a new charter and rules for the party. It was held in 1974 in Kansas City and featured a fight between organized labor and party "regulars" on one side, and reformers, women and blacks on the other, over the issue of affirmative action in delegate-selection and other party affairs.

A group of Democratic governors helped then-party chairman Robert S. Strauss work out a last-minute compromise that averted a serious blowup.

But Carter, who used the 1974 miniconvention to meet and woo potential supporters for his candidacy, has been notably cool toward holding another such convention this year.

His agents let the resolution mandating the mini-convention go through the 1976 convention, because at that point they did not want another floor fight with the groups pushing for such a session.

But White House representatives have worked for the last two years to restrict the size, scope and agenda of the Memphis meeting.

Ironically, some of these White House advisers now fear this may backfire, and that charges that Carter has manipulated the proceedings will dominate the affair.

The convention agenda is hardly inspiring. The parley officially opens Friday night with the traditional round of welcoming speeches by local pols, a $65,000 film extolling the virtues of the Carter administration, a speech by the president and a reception for him hosted by his allies among the Democratic governors.

Saturday is devoted to workshops on such weighty subjects as the Mideast and farm and inflation policy, with an evening fund-raiser, to which delegates will have to pay their own way, thrown in for good measure.

Before delegates get a chance to vote on a single issue on Sunday, they'll take part in a memorial service for the Rev. Martin Luther King Jr., who was assassinated in Memphis in 1968, and hear a speech by Mondale.

If the recent opening round of procedural meetings is an indication, the party leadership intends to deal with challengers with a firm, if not heavy, hand.

Party leaders, for example, gave up only two minor concessions to liberals during a rules meeting last Wednesday. One was to agree to begin debate on resolutions no later than 11 a.m. Sunday, thus assuring at least five hours for discussion of issues during the convention. The other was a promise to be flexible on debate time for some key issues. Currently, each resolution is limited to a 10-minute debate, five minutes pro, five minutes against.

Although chairman White indicated a willingness to compromise further on some other procedural rules, straw votes of the party's executive committee indicated that if he's so inclined he could effectively kill any challenge when the full Democratic National Committee meets to reconsider them this week.

Dissidents didn't fare much better in a second meeting last Thursday, called to decide what resolutions will appear on the official agenda.

Almost all of the 24 resolutions approved were innocuous, taking such noncontroversial stands as supporting family farms and Carter's Mideast policy.

Resolutions can also be brought to the convention floor by collecting the signatures of one-fourth of the delegates. But these petitions have to be presented before the convention begins and, under convention rules, won't be considered until after the 24 already on the agenda.

Just how much trouble the convention structure will cause among activists such as Judy McCarthy, a party newcomer from Arizona, is unclear. She complained to the rules meeting that it was "limiting my participation. We're all going to considerable time and expense to attend this convention and we want to be heard. Otherwise, what's the use of coming?"