CAN THE STATE of Washington cordon off its voters from interviews by members of the press? Not any more. That was the intention of a law passed by the Washington legislature in 1983, which prohibited anyone working for a news organization from getting within 300 feet of poll entrances; that would effectively prohibit exit polls of the type conducted by the television networks and some newspapers. The law was challenged in federal court by, among others, the Everett Herald, which is owned by the Washington Post Co., and it has now been ruled unconstitutional by Judge Jack Tanner.

The only thing wrong with the decision is that it came so late. Last year Judge Tanner upheld the law; his decision was reversed and the case sent back for reconsideration of certain basic questions. To them he gave the only sensible answers. No, there was no evidence that exit pollers had caused disruption at the polls. Yes, the secretary of state wrote the governor and told him that the purpose of the law was to prevent network projections. He might have added, yes, there was a lot of commotion in Washington and other West Coast states after the 1980 elections, when the networks projected results in states whose polls had closed, totalled up the electoral votes and declared Ronald Reagan the winner, and then broadcast Jimmy Carter's concession statement live.

There is a small point to be made here and a large one. The small one is that this was a petty, spiteful law. Should the networks have blacked out Mr. Carter's speech? Should they be barred from broadcasting election results that clearly indicate one candidate is winning and one losing? Obviously the answer to both is no. The problem, if there is one, is being addressed anyway by Congress, which is considering a measure by Rep. Al Swift (D- Wash.) to solve the problem through uniform national poll-closing times.

More serious is the reason this law is unconstitutional. It was urged by the state that exit polling is somehow "commercial" speech which can be regulated or effectively barred in the interests of some greater good, in this case to protect "the election process." But this is not the choice the Framers made when they adopted the First Amendment. There is always some supposed greater good to appeal to, and there is usually some commercial value to what is sought to be printed or broadcast. If a state can cordon off voters on Election Day because of injury to "the election process" or because the resulting stories might lead citizens to behave in ways legislators think undesirable, then why couldn't a state prevent journalists from talking to citizens before or after the election as well? There is no easy or comfortable place to stop once you start dwn the road the Washington legislature embarked on. Fortunately the courts have told the state to turn back.