The hoary issue of early network projections on Election Day has reared its head once again.

More than 150 members of Congress have signed a letter to the presidents of ABC, CBS and NBC, urging them not to project a winner in the presidential election until all polls have closed on the West Coast. "Every time a network projects while citizens are still voting, it sends a message that the votes of those still waiting to cast their ballots do not matter," it says.

Curtis B. Gans, head of the Committee for the Study of the American Electorate, has placed ads in today's New York Times and Los Angeles Times, urging people to flood the networks with complaints. "Don't let the networks preempt your vote!" the ad says. Studies have shown that some voters stay home if they hear a presidential winner projected.

Separately, Rep. Ron Wyden (D-Ore.) has asked President Bush, Bill Clinton and Ross Perot not to claim victory or concede defeat until all the polls have closed.

Network officials say they will not project winners in an individual state until most of its polls have closed, but that they will call the presidential race once their joint exit poll indicates that a candidate has the necessary 270 electoral votes.

"I don't think it's appropriate for us to withhold that information," said CBS vice president Joe Peyronnin, adding that Congress could have solved the problem by setting a uniform time for poll closings.

"I don't know any way around it as a news organization," said NBC spokeswoman Katherine McQuay. "If you know all the votes are there, do you withhold that information? Do you pretend there's still a horse race?"