VIRGINIA'S REPUBLICAN leaders have opted to hold a Feb. 29 presidential primary in hopes that it will attract their party's leading candidates between now and then. While the relatively early date may generate more candidate appearances, primaries in Virginia are odd gauges of partisan sentiment.

Virginians do not register by party, making primaries hard to handicap or even to assess when the results are in. You can't be sure how many "crossover" voters will cast ballots for a primary candidate they believe to be weakest in order to help the other party's candidate in a general election. For that matter, nobody can count crossover voters that well because if voters don't declare partisan preferences in the first place, what are they crossing over from?

To try to deter these invisible crossovers, the state board of elections has added a new, unreliable hitch: Before casting their ballots, voters must sign pledges stating their intentions to support the Republican Party nominee in the November election. That should make for some nifty investigative work; rewards for information leading to a conviction might be offered, followed by cash prizes for the best suggestion of a punishment.

The Democrats at this point are slated to conduct caucuses. "Our process may be less exciting," Craig Bieber, executive director of the state party, said in an interview last month, "but primaries are always a double-edged sword." But should the party decide to switch to a primary, how many calculating voters will try to be double-cross-overs?