Until last month, conservative groups were preparing a major promotional campaign to boost President Reagan's Central American policies: mailings, advertisements, speeches and television commercials designed to bring the disheveled national consensus into line and drag Congress along with it.

Then Reagan named former secretary of state Henry A. Kissinger to head a bipartisan commission on long-range U.S. policy for the region.

The right exploded. Now its thunder, in the form of a media assault beginning after Labor Day, instead will urge the public to instruct Reagan and Congress on true anti-communist behavior.

"We were ready to make a massive commitment on the administration's behalf and they went and pulled one of the dumbest political moves of any administration yet," said Paul Weyrich, who heads the Coalitions for America but is better known as chief of the new-right Committee for the Survival of a Free Congress.

The Kissinger commission is "the same people who gave us Vietnam and the Iran fiasco" and "a real slap in the face to conservatives," said Peter B. Gemma, president of International Policy Forum, a new group he described as "a sort of conservative Trilateral Commission."

The media campaign now will "stick to what we think is the right thing to do . . . rather than linking ourselves to the president," Weyrich said.

He said that the campaign could have an impact on the 1984 election campaign in the way similar efforts made the Panama Canal treaties a major issue in 1978 and derailed ratification of the SALT II treaty with the Soviet Union in 1980. By Election Day, 1984, Weyrich predicted, "people will be asking Reagan to do what we're asking him to do. It'll be the politically prudent thing for him to do it."

Central America has become a top priority for most older conservative organizations and has sparked the founding of several new groups.

Weyrich said at least 30 groups have been represented at weekly breakfast strategy sessions and at separate weekly briefings on Central America held by Faith Ryan Whittlesey, Reagan's public liaison officer. Many of these groups are offshoots of others and share officials, funding sources, mailing lists and office space.

Most of them are aiming now at the forthcoming educational campaign, which Weyrich said will push "a clear-cut policy of victory in Central America." As outlined by several participants, the program of ads, speeches, television programs and newspaper articles, to continue through the 1984 elections, will include:

A declaration that U.S. interests forbid further leftist advances in the region along with detailed descriptions of the Cuban, Nicaraguan and Soviet presence. "Polls show that if people understood about the bases there, that there might be a massive influx of refugees into the United States . . . there would be massive support" for a harder U.S. line, Weyrich said.

Opposition to negotiations with leftist forces as being a sellout. Although President Fidel Castro of Cuba, for example, is invariably called the source of Central America's problems along with the Soviet Union, he is also denounced as untrustworthy. Talks with him, Gemma said, would be like "the local police chief setting up detente with the Mafia commander."

Open support for efforts to overthrow the leftist Sandinista government of Nicaragua. "We want a policy that has as its objective the elimination of Soviet power from this hemisphere," said Howard Phillips, president of the National Conservative Caucus. "We worry that the Soviets will think they can keep Nicaragua as long as they don't take El Salvador until the 1984 U.S. elections are over."

A clear willingness to commit U.S. troops if it proves necessary. The central issue of the 1984 campaign, Phillips said, will be, "Shall America be defended, in Central America and in overall foreign policy? If Reagan doesn't put it that way, he'll be in the position of having to prove that he's not against blacks, against women or against poor people."

Two new groups are shouldering much of the effort:

The Citizens' Commission on the Crisis in the Americas, a 12-member conservative alternative to the Kissinger commission that Phillips and others formed to make its own report on long-term policy. Temporarily headed by James R. Whelan, editor and publisher of The Washington Times, the group includes Sen. Steve Symms (R-Idaho); Ron Godwin, executive director of the Moral Majority, and Lynn Bouchey, head of the Center for Inter-American Security.

The Central American Freedom Alliance, an ad hoc coalition of several conservative groups pooling funds for projects and publicity. Executive director Robert E. Baldwin said contributions and mailings, using lists supplied by noted Virginia conservative Richard A. Viguerie, have netted the group $20,000 in its first three months.

The group held a July 19 conference that drew 150 congressional staff members and others to hear U.N. Ambassador Jeane J. Kirkpatrick and several Nicaraguans push support for the anti-Sandinista rebels. Regular press mailings and briefings are scheduled to begin after Labor Day.

Ideas and policy proposals from established conservative think tanks--including the Heritage Foundation, the American Enterprise Institute and the Council for Inter-American Security--receive wide circulation in the Reagan administration, and many staunch rightists hold decision-making power on Central America: Kirkpatrick, national security affairs adviser William P. Clark, and National Security Council staff advisers Alfonso Sapia Bosch and Roger Fontaine, among others.

But conservatives of the so-called "new right," far from being happy at what liberals call their increasing power, are unanimous in complaining that Reagan "is now playing consensus politics with Central American policy," as Phillips put it.

"He's shaping his policy on the basis of opinion polls," Phillips said.

Kissinger's appointment has even made the right suspicious of old friends. "There's a concern that Kirkpatrick and Clark were the ones who put the commission together," said one influential conservative writer. "The fear is that the administration is going to go into a negotiating mode and eventually sell out the Nicaraguan freedom-fighters."

Otto Reich, Reagan's official coordinator of public diplomacy for Central America, acknowledged the conservatives' worry. "That's what the country is all about: the center, not the extremes," he said. "We are trying to build a national consensus."

Other administration officials shrugged off the criticisms, noting that the right has no alternative to Reagan. Weyrich disagreed: "There is an alternative. People can stay home. They may vote for him, but they won't work night and day the way they did in 1980."

Conservatives admitted that their attacks could help Reagan with the political center by making him seem relatively moderate, but they denied that the White House has encouraged conservative criticism.

"I'm not sure they understand that effect," Weyrich said. "They're not that smart."