DO PROJECTIONS by the television networks of winners in electoral contests lower voter turnout in states where the polls haven't yet closed? Not clear, say the heads of the three network news divisions, who were on Capitol Hill Thursday to testify on the subject. Politicians, particularly some defeated by close margins in 1980 in Western states where the polls were still open after Jimmy Carter made his concession statement, say they saw people leave the lines when they heard the news. It's possible that the news discouraged enough voters to make the difference in a few races. But that doesn't mean that Mr. Carter's statement should not have been broadcast.

Nor does it mean that the networks should not make projections of winners based on partial returns, sample precinct results and exit polls. Some critics of the networks argue that such projections, in contrast to reports of actual tallies of votes cast, aren't news, and therefore may not enjoy First Amendment protection. But you shouldn't have much trouble seeing the danger of a precedent that says the government can decide what's news and what's not. The networks voluntarily try to refrain from reporting results in a particular state when its polls are still open. They are careful not to declare a winner in the presidential race until they are certain that a candidate has carried a majority of the electoral votes. They can hardly be blamed if viewers draw the obvious conclusion at some point in the evening from accurate reports that Ronald Reagan has carried 17 states whose polls have closed and Jimmy Carter only three.

The politicians and critics who would muzzle the networks have not done much to pursue other solutions to the problems they decry. They complain that the networks sometimes project results in states in two time zones, but there is nothing to keep such states from closing their polls simultaneously in both zones. Nor is there anything to keep the states or the federal government from setting simultaneous poll- closing times across the nation, or having the polls open for the same 24-hour period across the nation. There is nothing sacred about our current hours for voting, which were set years ago to suit the convenience of quite a different society. True, it is difficult and sometimes dangerous to make rapid changes in our slow-moving electoral machinery. But it might be easier and would certainly be less dangerous to address any problems that exist in that way rather than by censoring the news media.