Amid rumbles of internal criticism of some of their own members of Congress, Democrats raced through the preliminaries today of a midterm party conference designed to set the themes for the fall campaign against President Reagan and his policies.

Although the seven presidential hopefuls and the 897 delegates to the party's third mini-convention, opening Friday, are expected to focus their fire on Republican economic policies, resolutions on the Middle East and nuclear arms control vied for attention today.

Elder statesman Averell Harriman won the first standing ovation of the meeting when he told the Democratic National Committee he hoped the party would declare "the absolute necessity" of a nuclear-weapons freeze.

"As one who negotiated with the Russians for 55 years," Harriman said, "I believe they are ready to join us in that kind of agreement."

The enthusiastic response to the former ambassador's statement presaged endorsement for the nuclear-freeze resolution. But party officials were expressing some concern about a statement on Israel and Lebanon drafted by DNC member and Washington political consultant Mark Siegel.

The statement said the Israeli invasion "has dealt a blow to international terrorism and greatly reduced Soviet influence in the region." It said the United States should support humanitarian efforts to aid the civilian casualties, but contained no hint of the criticism of the bloodshed in Lebanon that senators of both parties heaped on Israeli Prime Minister Menachem Begin during his visit to Washington earlier this week.

The nuclear freeze and Middle East resolutions will come up for debate Saturday at one of the seven issues panels, where delegates will frame statements for Sunday's final full session.

Friday's schedule is mostly oratory--from House Speaker Thomas P. (Tip) O'Neill Jr. and a parade of presidential hopefuls.

The harmony theme that party Chairman Charles T. Manatt has sought to impose on the precampaign gathering will keep any of the debate from spilling over to Sunday, when the delegates can vote only to "accept" the panel's statements.

A small group of liberal DNC members complained today that those rules make the meeting "a sham," where Democrats can say only, "Republicans are bad and we are good."

Their complaint was echoed by Republican National Chairman Richard Richards, who told a morning news conference here that the Democrats offered "no new ideas," but just the old formula of "tax and tax and spend and spend."

He also accused the Democrats of "lying to the public" when they charged that Reagan and the Republicans wanted to cut Social Security benefits.

But Manatt brushed aside Richards' effort to shoe-horn his way into the Democratic story as "tacky," and vowed to keep up the assault on the economic and social policies of the administration.

Under Manatt's firm gavel, the DNC moved quickly through its agenda, while the party's presidential hopefuls worked the gathering delegates. Former Florida Gov. Reubin Askew and Sens. Gary Hart of Colorado, John Glenn of Ohio and Alan Cranston of California all tried their hand at courting the delegates. Sens. Ernest F. Hollings of South Carolina and Edward M. Kennedy of Massachusetts joined them here tonight.

Former vice president Walter F. Mondale worked an election-year-like schedule. He began the day touring an auto parts factory and exchanging tales of woe about Reaganomics with about 25 unemployed auto workers at a union hall in North Philadelphia.

"I'm obviously thinking about running for president," he said. "But it's too soon to decide something like that."

The only cloud on the horizon was the expressed unhappiness about the voting behavior of some conservative House and Senate Democrats, who have supported key parts of the Reagan program.

Terry Herndon, executive director of the National Education Association and cochairman of a party commission on "platform accountability," told the DNC that "we have a problem of our own . . . because some Democrats have spurned the party platform and cast the decisive votes for Republican victories."

The commission is scheduled to hear testimony today from Reps. Kent R. Hance (D-Tex.) and John B. Breaux (D-La.), who have supported parts of the Reagan program, as well as from some of their sharpest critics in Congress and the party.

Manatt told the DNC he felt "a real concern" that the internal debate "will get a lot of attention in the press" and distract from the assault on the Reagan policies.