When President Reagan addresses Congress tonight at 8 on Central America, he will be embarking on a campaign of persuasion that his advisers say has high risks but also provides the hope of salvaging a policy that deeply disturbs an increasing number of Americans.

"He's giving the speech because he thinks the fate of American policy in the region hangs in the balance," a White House official said yesterday.

Administration officials acknowledge that voter sentiment runs strongly against increased U.S. involvement in Central America. They say the president has decided he will not be able to get the military and economic aid he seeks to shore up the El Salvador government unless the American people can be convinced that they have a vital stake in the region.

"There's a high risk to this but it's a higher risk not to do it," said one official. "We are on a downward roll with Congress right now. The president feels that the public is confused and doesn't understand what's going on in the region and he wants a forum to address the people."

At stake tonight is Reagan's ability to muster bipartisan support for his policies for some of the closest and poorest U.S. neighbors in the hemisphere. The threat of escalating regional violence and uncontrolled covert intelligence operations has prompted key congressional committees, with broad support in both houses, to challenge Reagan's basic approach to Central America.

White House officials said Reagan decided to make his case before a joint session of Congress to guarantee network television coverage. They contend that public support for his defense spending proposals increased after his nationally televised speech March 10.

But skepticism about U.S. involvement in El Salvador and Nicaragua is higher than on most other national security issues. Administration officials yesterday cited public opinion polls, which they said are similar to the findings of private surveys, indicating lack of support for Reagan's policies in Central America.

The most recent survey, reported April 12 in the Los Angeles Times, found that only 26 percent of Americans believe the U.S. role in El Salvador is "morally justified," compared with 49 percent who believe it is not. By 45 to 33 percent, voters said El Salvador is not vital to U.S. national defense. By 40 to 23 percent, they said involvement there "will not end in victory."

Sen. Paul Laxalt (R-Nev.), general chairman of the Republican Party and the senator considered closest to Reagan, said yesterday that Reagan's speech was an effort to win over the "uncommitteds" in Congress.

"I suppose that one risk would be . . . that the speech is so bellicose that maybe some of the undecided people say, 'Hey, this is going too far and don't Vietnam us,' " Laxalt added. "That, I would think, would be the only down side."

However, White House officials said that Reagan is well aware of the dangers of giving a warlike speech and that he intends instead to deliver a measured account of U.S. policy that one official described as "more a report than a speech."

"If I go up there and give an anti-Soviet speech, nobody will listen," one official quoted the president as saying.

Except for State of the Union speeches to joint sessions of Congress, Reagan has used this forum only once before, when he spoke on economic policy on April 28, 1981. The last time a president addressed a joint session of Congress on a national security issue was June 18, 1979, when President Carter appealed for ratification of the SALT II treaty. It was never ratified.

Despite the high hopes at the White House, the prospects facing Reagan policy in Central America seem similarly difficult.

"What you're asking Congress to do is cast votes for unpleasant propositions," said Rep. Dick Cheney (Wyo.), who heads the House Republican Policy Committee and is a strong supporter of administration policy in Central America. "This is a long-term engagement. It's a lot more than asking for a vote on a single appropriations bill."

In immediate terms, the "unpleasant propositions" are defined by the president's request to Congress to spend $250 million in military aid over the next 18 months in El Salvador and Honduras, the two most important U.S. allies in the region.

But the policy also is defined in secret terms, which the president is unlikely to address directly in his speech. For the past two years, the United States has been spending $20 million annually in CIA funds to support paramilitary forces who raid targets in Nicaragua from Honduran bases.

Last fall many of these forces moved into Nicaragua in hopes of gaining popular support and fomenting insurrection against the leftist Sandinista government.

Another $20 million is being spent on covert intelligence collection activities in the region under code names such as Royal Duke and Rounder Watch, according to informed sources.

Reagan's foreign policy advisers have justified their request for more military aid in Central America in stark and simple terms, saying it is impossible to fight armed insurgencies with land reform and social justice.

"Each element of the Central American problem is related," Thomas O. Enders, assistant secretary of state for inter-American affairs, recently told the Senate Foreign Relations Committee. "No amount of land reform, or open elections, or improvement in human rights will end the conflict in El Salvador if Nicaragua continues to fuel it."

Reagan's critics in Congress have responded in a similarly simple and direct fashion, saying the president has relied too heavily on military means and aggressive covert activities by the CIA rather than trying harder to achieve negotiated peace in the region.

"The administration seems to have a penchant for substituting covert activity for foreign policy and, ultimately, that's not going to work," Sen. Patrick J. Leahy (D-Vt.) said in a recent interview.

"The administration is fast reaching a point that they're going to have to determine what will be their long run foreign policy and they're going to have to seek a bipartisan consensus or, ultimately, the Congress is either going to impose a fragmented one of its own or blow the whistle on all the covert activity and the administration will be left with a shambles."

This is what Reagan will be seeking to avert with his speech tonight. Administration officials said he will make some concessions, but he basically will be seeking to mobilize members of Congress and voters who have not made up their minds behind some variant of his policy.

"In the president's view, either the Congress is going to join with him to support the chances for economic growth and freedom in the region, or Congress is going to turn its back and watch the chances dwindle away," said one White House official yesterday. "The situation now is reasonably manageable and the costs are small. The aim is to deal with it now before it becomes a crisis.