ONE OF THE SMALLER schools of political conspiracy holds that every four years the Opposition Party trains and dispatches militant hordes of its voters into "Our Party's" primaries where they fiendishly force "us" to nominate the least-electable of "our" candidates and thereby ensure the Opposition Party's victory. As conspiracies go, this one is not very popular, because nobody has ever defected from the Opposition to reveal its training manuals or techniques -- and because people who are voters do not work that way. They almost always vote for the candidate they like most, or perhaps dislike least.

While they are not part of an organized or disorganized conspiracy, "crossover" voters have become a major political issue in the Republican campaign this year. They were the basis of Rep. John Anderson's strong showings in both the Massachusetts and Vermont primaries. His supporters trumpet the crossover votes as an indication of his general election appeal to Democrats and independents. But his primary opponents, whose interest is neither objective nor academic, have questioned the propriety of those crossovers and especially Mr. Anderson's overt importuning of non-Republlican voters to vote in Republican primaries. Ronald Reagan, the Republlican front-runner, makes a rather self-serving distinction between "good" crossover voters in states like Georgia, Alabama and South Carolina and "bad" crossover voters in states like Massachusetts and Wisconsin.

Maybe we are missing something in the discussion, but the question seems to be a direct one: should non-Democrats (or non-Republicans or non-Communists) participate in and influence the party's most significant decision -- the nomination of its presidential candidate? While we are not inclined to romanticize or go all soggy over the political parties, we do believe that this country's two major parties are essential to effective government and especially, at this time of political fragmentation and single-interest politics, to effective leadership. In the American system, parties have been the natural and logical -- and principal -- place where coalitions are built and public agendas determined and power distributed. It is not an overstatement to say that generally, when parties are strengthened, the political process works better.

But to mean anything, a party must engage the interest and energies of members who believe in the group's objectives and care about its fortunes. It does not seem too much to ask of people who are participating in the party's most important single act -- the nomination of a presidential candidate -- to declare themselves to be party members, to indicate that their interest in this institution continues beyond the fate of Candidate X. The political parties need all the help they can get, and that is not provided by casual or trendy one-shot voters in a presidential primary.