The dispatch of Philip C. Habib to the diplomatic front in Central America is an apparent effort to clothe an old administration policy in new diplomatic dress in order to mollify an increasingly critical Congress.

It is the latest repetition of a pattern the Reagan administration has followed whenever Congress notices anew that efforts to make the leftist Sandinista government of Nicaragua cry uncle involve military action and the killing of human beings. When Congress protests, the administration offers a diplomatic gesture to pacify the lawmakers.

In November 1981, a National Security Council document outlined plans for broad "political and paramilitary operations" against Nicaragua, a policy adopted after the administration apparently concluded that the Sandinistas were a Soviet tool and would have to be compelled to abandon their desire to spread communism.

For a long time, however, the administration's public defense of its policy was very different. In 1981 the White House argued that aid to the contra rebels would serve only to stop Nicaragua from arming leftist guerrillas in El Salvador. It has come a long way since, as indicated by President Reagan's warning this week that defeat of new aid to the contras would produce "a sea of red, eventually lapping at our own borders."

In the beginning, Congress was concerned about El Salvador and most members heeded the administration's declaration that Nicaragua was a threat there. But lately key moderates in Congress have become convinced that real negotiations with Nicaragua have never been tried.

Their resistance to more military pressure has peaked just when the administration has decided nothing else will work. Now, the White House evidently hopes that the appointment of Habib will persuade Congress to provide him with a $100 million club he can use to threaten Nicaragua into submission.

Asked yesterday what negotiating tools Habib would have that his predecessors lacked, State Department congressional strategist Richard N. Holwill replied, "I hope to God he has lethal aid to the contras . There's nothing else the Sandinistas want from us" except an end to contra attacks, he said. "If we don't have that, we don't have negotiations; it's that simple."

The 1981 decision document said covert U.S. efforts would try to "build popular support in Central America and Nicaragua for an opposition front that would be nationalistic, anti-Cuban and anti-Somoza." New "action teams" of rebels would support that front.

But when Central Intelligence Agency Director William J. Casey briefed congressional intelligence committees about the newborn "contra" rebel aid program in December 1981, he did not suggest any political goals, according to participants. He said the objective was to stop Nicaragua's arms supply to Salvadoran guerrillas.

Vernon A. Walters, then an ambassador-at-large, described this in another context, at a November 1981 conference in Santiago, Chile: "Too often, we have announced publicly what we're going to do or what we're not going to do. We believe that constructive ambiguity is a very powerful weapon."

The contras grew from a ragtag total of 500 or so fighters in late 1981 to 4,000 a year later, according to intelligence officials, and the CIA pooh-poohed rebels' declarations that they were working to overthrow the Sandinistas.

But Congress was alarmed. In December 1982 Congress forbade spending "for the purpose of overthrowing the government of Nicaragua or provoking a military exchange between Nicaragua and Honduras."

In what would become a pattern, the CIA responded in February 1983 by telling the House intelligence committee for the first time that more aid to the contras was justified "to put more pressure on the Sandinistas to make them come to the bargaining table," according to a legislator involved.

Reagan declared in April 1983 that "anything we're doing in that area is simply trying to interdict the supply lines." He added, "I don't think it's reasonable" to think such a small guerrilla force "could nurse any ambitions that they can overthrow that government."

In May 1983 rebel strength reached 7,000, and they were hitting granaries, ranches, electric lines. "We'll be in Managua in five months," one said, referring to Nicaragua's capital. Reagan for the first time called the contras "freedom fighters."

"The definition kept changing of what the objectives were," recalled Sen. Daniel Patrick Moynihan (D-N.Y.), former vice chairman of the Senate intelligence committee, in a 1983 interview. Chairman Barry Goldwater (R-Ariz.) demanded that Reagan "tell us in plain language just what it is he wants to do relative to Nicaragua."

Again responding with diplomacy, the administration in June 1983 named former Florida senator Richard Stone to be its first special negotiator for Central America.

In July the House barred funds for "direct or indirect support of military or paramilitary operations" against Nicaragua. In August the White House named former secretary of state Henry A. Kissinger to head a bipartisan national commission on Central America.

The pattern held the following year. The contras mined the harbors of Nicaragua with CIA help, and damaged seven ships. Congress erupted in fury in April, and in June 1984 Secretary of State George P. Shultz visited Managua to open direct bilateral talks.

Last year Reagan acknowledged that he wanted to make the Sandinistas "cry uncle," and Shultz warned that failure to fund the contras would be "consigning Nicaragua to the endless darkness of communist tyranny."

When it became evident last spring that Congress would reject Reagan's renewed military aid request, he offered -- unsuccessfully -- to delay using the aid if Nicaragua negotiated with the contras.

Habib's mission is the latest twist in this plot, but the goals have not changed. "Let there be no mistake," Reagan said yesterday. "Ambassador Habib's efforts to achieve a diplomatic solution must be be accompanied by an increased level of [military] pressure on the Nicaraguan communists."