THE POLITICAL SIGNS in Lafayette Park raise a question that always seems difficult in the abstract, but turns out to be simpler when you look at the actual scene. Most of the signs face Pennsylvania Avenue and the White House, and some are substantial structures. Several are eight or 10 feet high. They convey a variety of messages, frequently supporting the causes of world peace and disarmament. The First Amendment certainly protects the right to express those views. Does it protect the right to express them on billboard-size signs parked permanently among the flower beds and fountains of a much-visited park? As the signs get bigger and more numerous, the idea of an unlimited right gets more dubious.

That block of Pennsylvania Avenue, and the park, have always been a locus of choice for people with messages to deliver to their government. A decade ago a Texan who had been carrying on a dispute with the Veterans Administration ever since World War II took to sleeping in the park as a protest against a ruling on his disability benefits. The Court of Appeals accepted the argument that sleeping in the park could be political expression. Then the Community for Creative Non-Violence attempted the same thing on a larger scale, pitching 20 tents in the park to protest inadequate housing for the poor. The case went to the Supreme Court, which held last summer that the National Park Service had the power to prohibit camp-ins whether they were political expression or not. Last year the Park Service also turned its attention to the rich mixture of signs, banners and vigils along the White House fence. The Court of Appeals upheld a precise and strict set of regulations, partly on aesthetic grounds and partly to protect the White House's security. But sweeping this multitude of messages from the south side of the avenue has increased the pressure on Lafayette Park on the north side.

Free expression and dissent tend to be untidy, and a certain amount of untidiness has to be tolerated to accommodate them. They are, after all, essential. But there have always been limits. That's not new. You don't have to be told that the courts would not encourage you to spray-paint your constitutionally protected opinion of your congressman on the Washington Monument.

For Lafayette Park, the Park Service has now proposed new rules. They would continue to permit demonstrations there. They would permit you to carry signs there. They would permit you to set up a stationary sign as large as four feet square, as long as you stay there with it. These rules are chiefly aimed at the huge, four-season, all-weather signs that have lately taken up permanent residence. These rules are reasonable, and the time has come when, unfortunately, something like them is necessary.