It is not all that common, even in the strange world of extremism, to find the Marxist-Leninist Party and the John Birch Society agreeing about what the voters should do on Election Day. However, this year those two organizations have the same recommendation.

Both the Marxists and the Birchers have told their followers to boycott tomorrow's elections. Their reasons boil down to the old joke: "Don't vote -- it only encourages them."

If the pollsters and political scientists are right, about 40 percent or more of the people old enough to vote will follow that advice by not voting Tuesday. If those who do turn out split fairly evenly between the two major parties, the big bloc of people who don't think voting is worth the trouble will constitute the largest political party in the country.

Voter participating in general elections has been on the decline for the past two decades (although it is higher today than it was through the first three decades of this century).

The Washington-based Committee for the Study of the American Electorate issued a report Friday noting that various factors -- including a decline in registered voters in some states -- suggest a lower turnout this year than in 1976. But that study did not include registration figures from four big states -- California, Illinois, Texas, and Massachusetts -- where election officials predict higher participation this year.

Get-out-the-vote coordinators of both major parties say there is a chance that the percentage of people who cast a vote will increase somewhat this year, reversing the trend of the past four presidential elections. But there is not much chance, the demographers say, that the participation rate will be much above 60 percent. Roughly 85 million adult Americans will vote tomorrow; roughly 75 million will stay home.

This phenomenon has been studied extensively by scholars and politicians of every persuasion, but there remains no consensus on why it is so or even what it means.

The decline in voting has come at a time when the level of education was increasing and legal barriers to voting were being pulled down. Voting rates have dropped at the same time that other, more demanding forms of political participations (like writing a check to a candidate or party) are on the rise.

Even the numbers on this subject are confusing. The standard method for determining how much of the eligible electorate votes comes from the Census Bureau, which does the obvious thing -- it divides the total presidential vote by the total voting-age population. But social scientists and census officials agree that this method gives a misleadingly low percentage.

The Census Bureau calculates, for example, that the 81.6 million Americans who voted for president in 1976 constituted 54.3 percent of the voting-age population. But the "voting-age population" is a misleading base because it includes several million aliens and other people not eligible to vote. And the vote for president is not an accurate measure of turnout because 1 million voters in 1976 passed up the presidential line on the ballot. b

Figuring in variables like these, Richard Scamon, a former director of the Census Bureau who now runs the Elections Research Center, says the voting particiption rate in 1976 was closer to 65 percent.

That kind of adjustment makes the U.S. voter stack up a little better compared to counterparts in the Western European democracies, where turnout generally runs from 70 to 95 percent. But it still does not explain why voting participation has been dropping in every election since 1960, when the turnout (using census figures) of 62.6 percent was the highest since before women were given the vote in the 1920 election.

Why don't people vote? Many voters are simply turned off by the choices the major parties have provided. Arlene Freise, an office worker in Los Angeles, speaks for this state of mind. "You know," she said the other day, "I've thought about it and I've decided that [President] Carter and [Ronald] Reagan are both turkeys."

If you need more evidence, you might ask Barry Commoner, an otherwise little-notice presidential candidate who aired a radio commercial two weeks ago that summarized the Carter and Reagan campaigns in a single word -- an off-color word often abbreviated as "B.S." Commoner figured that word would create an angry reaction, but he needed to get attention some way, so he went ahead.

There was reaction, all right -- but not so angry. To a large extent, the people who responded to Commoner's ad agreed with him. The tone of the newspaper editorials was reflected in the headline over a column in the St. Paul Pioneer-Press: "Well, He Was Right, Wasn't He?" A reporter in Los Angeles called 20 people at random and asked them to complete the sentence, "The presidential election campaign so far has been mainly ------." Twelve answered with the same word Commoner used.

This did not surprise Curtis Gans, director of the Committee for the Study of the American Electorate, who made the prediction Friday that voter turnout this year will continue the 20-year downward trend. The reason, Gans says, is "disillusionment, alienation, and a decision that the reaction to a load choice is not to choose."

Before you lose sleep over all the alienation afoot in the nation, however, you might want to listen to Everett Carll Ladd, a political scientist at the University of Connecticut. Ladd argues that "the overall record of western democracies . . . better fits the linkage of low turnout to relative voter satisfaction than to widespread political alienation."

Political scientists who share this view tend to compare the United States to Switzerland, a stable, peaceful, generally contented society where voter turnout is often lower than it is here. And they note that opinion surveys usually show that people who are "alienated" -- that is, people who say the parties and the candidates are unsatisfactory -- split about evenly between voters and nonvoters.

What the polls show is that the basic reason people don't vote is that they don't think their vote will make a difference. Some people in this category may think both major parties are beneath contempt; others may feel that the two are both pretty good. They all seem to agree that similarites among candidates and parties are greater than the differences.

But while a big chunk of the adult population of the world's oldest democracy has clearly tuned out on the democratic process, there are lot of signs that those people who are still involved are more involved than ever.

Studies of various forms of political participation -- making contributions, signing petitions, writing letters to public officials, going out to a political rally, working for a candidate or a party -- all indicate that more of the population is engaged in more political activity today than at any previous point in U.S. history.

A reporter traveling with candidates does run into lots of people who are fed up with the system -- but also meets hundreds of Americans who have taken time out from jobs, homes, and hobbies to work for their political goals. The Republican National Committee says it recruited 800,000 volunteers this fall to run voter-registration drives around the country. Hundreds of thousands of others are out working -- in almost every case without pay -- for candidates for offices ranging from president to drain commissioner.

There are other signs that any reporter covering politics this year would have noticed: Ann Riley, a nurse in Berlin, N.H., who worked as a volunteer in the Kennedy campaign and used up all her vacation time on political work and then called in sick for the next 25 days so she could spend her time driving her own car around the state to help her candidate. Or Marvin Wanetick, a cab driver in Detroit who supports Commoner's Citizen's Party. He lost his job this fall but found a silver lining: "At least it gives me more time to work on politics," Wanetick said.

Or the 30 members of the Libertarian Party in Montana who gathered at a Sunday night dinner last month to honor the party's presidential candidate, Ed Clark.

People had barely started their meals when the state chairman stood up and announced that everyone present would have to pay for their own dinners. With some grumbling, everybody paid. Then the chairman reminded everybody that Clark needed campaign funds; more grumbling, but everybody contributed something. Then the chairman stood up and announced that, if the party failed to come up with a $520 deposit by Monday morning, the state party would lose its telephone. He pleaded for contributions.

This prompted even more grumbling, and at this point Clark had to leave to move on to the next campaign stop. I left with him.

Three days later, I started wondering whether those 30 people could have come up with an additional $520 for a political organization that has almost no chance of winning an election any time soon. I dialed 406-728-0847. A man's voice answered in a strong, confident tone:

"Montana Libertarian Party," he said.