A disinformation and scare campaign is going on in Washington these days as President Reagan struggles to win approval of his $100 million aid proposal for the Nicaraguan rebels, and the Sandinistas are not the principal culprits.

In its fervor to arm dwindling rebel forces before they are overwhelmed by Sandinista troops equipped with Soviet helicopter gunships, the administration has raised arguments rarely heard since the divisive days when opposition to the Vietnam war was equated with lack of patriotism.

First, William J. Casey, director of the Central Intelligence Agency, flashed a report purportedly describing a sophisticated Sandinista disinformation campaign. He then declined to release the report and left the unsavory impression that those who oppose the aid package are Sandinista dupes.

This was followed by a frontal attack led by Patrick J. Buchanan, the White House director of communications. He is no pussyfooter. In his view, as expressed in The Washington Post, the Democrats are continuing a "withdrawal from the great western coalition to contain communist expansion," which began in Vietnam, and are now serving, "with Moscow," to guarantee Soviet expansionism in Central America.

Buchanan, out in front of his president on some issues, is right in step on this one. Reagan argued last week that rejection of his package could mean "a sea of red, eventually lapping at our own borders." The president believes that the red tide will crest in Harlingen, Tex., while Buchanan sees it coming ashore at San Diego.

Another part of the effort to frighten members of Congress into voting for a package suspect on its merits is systematic discrediting of respected inquiries showing a pattern of atrocities and human rights abuses by the rebels, known as contras or counterrevolu tionaries. The administration used to acknowledge these abuses and contend that they were of lesser magnitude than those committed by the Sandinistas. This argument is too refined for the scare campaign, and contra crimes are now dismissed out of hand.

Last week, Americas Watch issued an even-handed report documenting murders on both sides and disputing the basic Sandinista alibi that Nicaraguan repression is a response to military pressure. But the report also found that contras "repeatedly sought out those they considered to be Sandinista activists and mutilated and murdered them." While contra military activities decreased in 1985, their human rights abuses did not.

White House spokesman Larry Speakes, asked about the findings, said, "We don't place much stock in that report . . . we don't think it's that reliable." While administration officials acknowledge privately that the report's reliability is not in question, they don't want to deal with contra abuses because this could undermine the scare campaign.

Not all the scaring comes from the White House. Without evidence, some Democrats view aid to the contras as a step in a Reagan plot to use U.S. troops in Nicaragua. Neither Reagan's statements nor his track record supports this notion.

Except in lightly defended Grenada, Reagan has been reluctant to deploy the military power he extols. He withdrew the Marines from Lebanon while pledging undying support for the Lebanese government they left behind. When Alexander M. Haig Jr. was secretary of state, he wanted to "go to the source" and nail Cuba. Reagan didn't buy it. A few disputes later, Haig wound up going to the private sector instead.

But Reagan, who has helped create a national era of good feeling, risks more than legislative defeat in his current hardball campaign. By equating opposition with tolerance for communist expansion, he debases legitimate debate and encourages a return to the ugly divisiveness of Vietnam days. Even if this strategy works in the short run, it is unlikely to build a long-term consensus for containing the Sandinistas.

As Buchanan observes, Soviets and Cubans are not in Nicaragua to pick bananas. But that is not an automatic argument for aiding murderous guerrillas of uncertain military potential. Reagan ought to find a better way of framing the issue when he takes his case to the country on national television Sunday.

Reaganism of the Week: Asked at a breakfast with reporters last Wednesday whether luck had played a role in the success of his administration, the president replied: "If I were really lucky, I wouldn't have this job . . . I'm safe to play cards with."