President Reagan is under increasing congressional pressure to spell out his wants and hopes for Central America in a low-key manner, especially from supporters who worry that Congress' slow and unsteady movement in his direction may soon halt.

Critics also want Reagan to be more specific, claiming that the policies' internal contradictions then will be more obvious to the watching world.

In the wake of the president's much-praised April 27 speech on Central America, Congress has given him most of the aid funding he has requested, albeit with much foot-dragging.

Two months ago, for example, debate on foreign aid requests included demands for proof that Nicaragua was supplying arms to leftist guerrillas in El Salvador. This month, there was no dissent when the House Foreign Affairs Committee denounced Nicaragua for exporting revolution.

Similarly, for the first time since military aid became an issue, no member of Congress suggested a total cutoff of aid to El Salvador. Yet the constant hesitation over aid levels has occasionally stalled supplies in the pipeline and has contributed to Salvadoran uncertainty on the depth of the U.S. commitment, according to administration and congressional officials.

The U.S. public is also uncertain, according to recent polls, which generally show widespread ignorance about Central America and deep mistrust of Reagan's push for more U.S. involvement there, the officials said. Republicans want the president to lead an educational effort but cool his rhetoric.

"No one in the House or Senate wants to be tagged as losing El Salvador because we haven't gone to the full amount" of aid requested, said Sen. Nancy Landon Kassebaum (R-Kan.), whose moderate views are influential on the Senate Foreign Relations Committee. "But we ought to have it fully explained why and to what use the aid goes and what makes it so important, why they would lose without it. The public has to understand it."

She and others cautioned that more rhetoric will not do the trick. Telling congressmen, as the president did Friday, that failure to grant his current $80 million request will mean the loss of El Salvador to communism "just makes it all seem too simple. It's misleading," Kassebaum said.

A key Republican Senate staff aide, self-described as a "critical friend" to Reagan's policies, said such dramatic appeals "remind people of Vietnam. It's saying every time, 'Just give us this, and it'll all be okay.' But it's never just that, it's always another step."

A State Department official deeply involved in implementing Reagan's policy in the region agreed that the sales pitch needs reworking. "There's no doubt about it," he said. "We've had too many flights of rhetoric that have little to do with actual policy."

He complained that Congress fosters such oratory by balking at higher economic aid levels unless the request is bolstered by evidence of military dangers, even though nearly everyone agrees that economic chaos is at the root of Central America's problems. There is much bipartisan support for legislation establishing a kind of Marshall Plan of economic aid for the region, but that is seen as a long-range plan for an area now in flames.

Democrats and administration critics are virtually daring Reagan to go into detail on his current scenarios. "Sooner or later, his logical fallacies will come back to haunt him," a key Democratic member of the House Intelligence Committee said.

He and others said the fallacies are twofold:

* Reagan has called Nicaraguan aid to Salvadoran guerrillas a threat to U.S. national security while saying he has no plans to use military force in the region. "What is military force for if not to combat national security threats?" the congressman asked. "He can't have it both ways."

* Reagan warns that without further U.S. aid, communism will spread through the area, driving a huge wave of refugees toward the U.S. border. Yet higher U.S. aid will intensify the fighting and further decimate the countryside, which also produces more refugees, the congressman said.

Administration officials are annoyed at these criticisms. "One wonders how Reagan could be more public than he has been," a White House official said. "He's spoken out at every opportunity to raise public awareness on the importance of this issue." The April 27 speech, the official said, is fully detailed and free of contradictions.

Reagan's defenders acknowledge that support is hard to find within the community of academic experts who write decision-makers' background studies. "That's partly because the administration has had a difficult time explaining its policy," said Richard Araujo of the Heritage Foundation, a conservative think tank.

Things have improved since the April speech and since top State Department officials for the region were replaced, Araujo said. "Now the president directs policy. Those who don't agree are not going to be players," he said. That means Salvadorans no longer will receive conflicting U.S. messages and should become more unified in their efforts, he said.

The best that can be hoped, even if Congress gives Reagan everything he wants, said Araujo and other supporters, is that the Salvadoran armed forces will push the guerrillas into enclaves and contain them while the rest of the nation holds a free election for the presidency. Then U.S. aid can help rebuild the shattered economy.

Critics and other Latinists disagreed, saying time favors the guerrillas even if Reagan gets his way, because unchecked government human-rights abuses will alienate the population while the guerrillas continue terrorist disruption.

"All the options are terrible," said Richard L. Malett of Southern Illinois University, vice president of the Midwest Conference on Latin American Studies. "Reagan doesn't grasp that the governments down there are not conservative in the Barry Goldwater sense. Their tactic is generally to shoot anyone who might ever have a radical idea."

Malett and most others said massive U.S. military intervention is probably not in the cards. "The reality of an election next November means he won't send troops," Malett said.

Richard E. Feinberg, a senior fellow at the Overseas Development Council and author of the book "The Intemperate Zone," which deals with U.S. policy in the Third World, said a major military push would not work any better in Central America than it did in Vietnam.

"He may be less interested in winning in Central America than in picking up electoral support in the parts of the country that favor a hard line," Feinberg said.

Critics and supporters predicted, as a House Republican's key staff aide put it, that "the administration will get to the point where what they say will be down the policy line but what they do will involve making contacts" with guerrilla forces in an effort to reach a solution.