Flying the banner of "new populism," the organizers of "citizen action" groups in about 25 states met here this weekend to challenge those in the Democratic Party who say it has to adapt to a more conservative political climate to survive.

About 1,000 organizers of grass-roots environmental, consumer, farm and energy groups assured each other that old-fashioned door-knocking could repulse President Reagan's Republicanism and rescue the Democrats from what one labor leader called "the gutless wonders" in their leadership.

The occasion was the annual summer retreat of the Midwest Academy, a Chicago training-school for many of the professional organizers of the left. The theme this year -- following Reagan's landslide reelection last November -- was not retreat but "Charge!"

Two freshman senators who ousted Republicans last year, despite the Reagan tide, told the activists that the Democratic Party would dig its own grave by trying to adopt a more conservative approach. In almost identical phrases, Sens. Paul Simon (D-Ill.) and Tom Harkin (D-Iowa) said voters don't want the Democrats to be an imitation-GOP, they want "the real thing" -- a party that fights for its traditional concerns and constituencies.

The Rev. Jesse L. Jackson, a defeated 1984 contender for the Democratic presidential nomination, heaped scorn on the "mainline politicians" that he said "are making another radical shift to the right . . . seeking the new center or the new patriotism."

Ridiculing the argument that Democrats cater to "special interests" when they advocate such causes as the Equal Rights Amendment, environmental protection, stiffer corporate taxes, civil rights, affirmative action or aid to agriculture, Jackson said, "Those 'special interests' are the new majority."

But no one was harsher in condemning the trends in the Democratic Party than Owen Bieber, president of the United Auto Workers, one of the most politically active unions in the Democratic fold.

Calling them "gutless wonders," Bieber said, "The mainstream Democrats are abandoning us in droves," thinking they "can win in 1986 by dumping their so-called special-interest baggage."

The UAW president said they were victims of "Ronald Reagan propaganda" in thinking that the party had lost in 1984 because it was "soft on Jesse Jackson or the unions."

Although none of the speakers mentioned their targets by name, they made it plain that they included the new leadership in the Democratic National Committee and those governors and members of Congress who have called on the party to repudiate its links to "special-interest caucuses" and return to the political "mainstream."

Ridiculing those revisionist Democrats, Harkin said, "You don't win the hearts and minds of the American people by telling them you have lost yours and could they please point you in the right direction."

Harkin said grass-roots activists have to remind the Democrats that "the biggest problem we have in this country is that too few people hold too much economic and political power."

Texas Agriculture Commissioner Jim Hightower, a leader with Harkin and Simon in trying to launch a "populist caucus" of federal, state and local Democratic officials, said that instead of "hunkering down in the middle of the road," the party should aim its appeal at "anti-establishment malcontents who don't like the big boys in business, in banking, the government or the press."

"They're radical in their belief that equality of opportunity requires real decentralization of power," Hightower said of these voters, "and they can be ours if we go after them."

The citizens' action groups -- which go under such names as the Illinois Public Action Council, Massachusetts Fair Share and the Ohio Public Interest Campaign, have their antecedents in both the civil rights and labor movements. They recruited a new generation of organizers in the 1970s on such issues as toxic wastes, utility rates, Vietnam and nuclear war.

Many of the groups use nightly door-to-door canvassing to raise funds and recruit members, and they claim to have more than 1 million members and supporters. As the organizers have advanced into their 30s and 40s -- a phenomenon noted by the number who showed up here with infants and small children -- more of them have applied their skills to political campaigns.

There were ex-organizer legislators from California and Connecticut and city council members from several cities mixed in with people who run campaigns against phone companies and utilities with toxic waste dumps.

As a counter-force against the powerful pressures pushing the Democratic Party to the center of the spectrum, their contention is that they represent a new and growing grass-roots political machine.

Although they command the full loyalty of few federal officials -- even Simon disappointed them by backing the balanced-budget amendment -- they seemed ready this weekend to challenge the emerging orthodoxy of the Democratic Party.

Bieber put it this way: "We're not going to elect people with left-of-center political philosophies on campaign techniques alone. But if we spend nine-tenths of our energy organizing on grass-roots issues and one-tenth on campaigns, the politicians will come to us."