In the spring of 1986, with Congress in the midst of a battle over President Reagan's request to provide $100 million in military and humanitarian assistance to the Nicaraguan contras, groups supporting the administration ran newspaper ads in the district of Rep. Richard H. Stallings (D-Idaho) urging voters to pressure him to back the president.

The letters poured in from around the district, one of the staunchest pro-Reagan areas in the nation. But by a ratio of about 2 to 1, the letters urged Stallings to vote against contra aid. He voted twice that year to halt aid to the U.S.-backed rebels fighting the Marxist Sandinista government of Nicaragua.

"It really gave me cover," said Stallings, who won his congressional seat in 1984 by just 133 votes at the same time that almost four out of five of his constituents were also voting for Reagan. "If anything, it strengthened my own feelings which I sometimes have to submerge in that district."

Undaunted by that outcome, pro-contra groups are again running ads in Stallings' district as Congress prepares for a showdown vote on Feb. 3 that could end the U.S. role in supporting the rebels, a program that has been the centerpiece of Reagan's foreign policy in Central America.

For Stallings and perhaps 50 other House members, Democrats and Republicans alike, the next 10 days will bring a barrage of letters, telegrams, personal visits, television and radio ads and other forms of lobbying from groups on both sides of the emotionally charged issue.

But the effort, which is likely to cost millions of dollars, may have a marginal impact on the votes of those key House members who, to varying degrees, are undecided on the issue.

Because Congress has fought over this issue so many times before, close to 400 members of the House are firmly locked into their positions, about half for and half against contra aid. The issue -- usually decided by about a dozen votes -- will most likely hinge less on outside pressures than on how those 50 lawmakers interpret the president's request and events in Central America.

"It's a strange issue," said one Democratic leadership aide. "You are talking about 50 people. Everyone is focused on this group and how to win their support. But grass-roots activity doesn't have much impact. Those 50 people are very well informed; they know the subtleties of the issue."

Nonetheless, groups that have been organizing around this issue for years and that view Feb. 3 as a watershed date are still geared up to make one final all-out assault.

Rosa DeLauro, executive director of Countdown 87, an umbrella organization that has coordinated anti-contra-aid activities by more than a dozen peace and political action groups, expects the campaign to cost

$1 million.

Since it began in June, Countdown 87 has generated tens of thousands of letters to lawmakers, conducted polls in the districts of key swing voters, run television and radio ads and conducted focus groups in an organizing blitz that DeLauro calls "unprecedented."

Another umbrella organization opposing contra aid, Days of Decision, plans to marshal thousands of people around the country today to protest the aid request that Reagan is expected to outline in his State of the Union address tonight.

The events will range from vigils, to fasts, to draping banners over highway overpasses, to a flyover of Orange County, Calif., by a plane towing a banner saying "Save Lives -- No Contra Aid."

"The reason the vote is so crucial to us is that if aid does pass, you really will see the effective end of the peace process in Central America," said Bill Spencer, a Days of Decision staff member.

Conservative groups agree that the stakes are high. "This clearly is the single most important vote," said Francis Bouchey, president of the Council for Inter-American Security, a conservative think tank in Washington. "The question is really not a matter of war or peace in Central America. It is a question of communism or democracy."

For both sides, the tradecraft is much the same. Bouchey estimates the council will spend more than $1 million on a campaign that will crescendo over the next 10 days with newspaper and radio ads and more than 2 million pieces of direct mail.

The group has even run newspaper ads in the district of House Speaker Jim Wright (D-Tex.) labeling him "Commandante Wright."

What effect the barrages will have on those lawmakers in the middle is a matter of dispute.

Rep. Olympia J. Snowe (R-Maine), who in 1986 was one of the handful of House members who switched their positions to give Reagan his victory on contra aid that year, said she is keeping "an open mind" and will base her decision on events in Central America.

"This issue is extremely polarized in Congress," added Snowe. "Very few members are in the middle on this issue, so it really depends on what emerges in these {peace} talks."

But Rep. George Miller (D-Calif.), who serves as the national spokesman for Countdown 87, argues that on such a close issue grass-roots activity can make the critical difference.

The group's polling data can persuade members fearful of reprisals that their constituents are strongly opposed to continuing aid to the contras, Miller said.

"I've had several members we've targeted come to me and complain that their people are getting agitated," said Miller. "I told them 'that's the point.' "