For reasons connected with the high cost of parking at the fancier Washington hotels, my occasional early-morning walking route to a press breakfast takes me across Lafayette Park, just opposite the White House.

Here, where Henry Adams once built a great house, and where Andrew Jackson still rears his horse in equestrian splendor, my dedication to the constitutional right of petition undergoes -- and invariably flunks -- a stern test. It is not unlike the test your belief in free speech would undergo if someone were screaming political slogans in your ear every time you hit the sidewalk.

The test for me is the clutter of billboards, placards, tents, mock-cemeteries and whatnot that now disfigures one of Washington's most agreeable squares, and one of the few refuges of distinguished architecture.

I was delighted, therefore, to read that the National Park Service intends to crack down on the demonstrators who (often in absentia) have turned Lafayette Park into a junkyard, a zealot's haven but a citizen's eyesore.

New regulations would restrict the size of placards -- a long, ugly row of which now conceal, at eye level looking across Pennsylvania Avenue, the north facade of the White House. They would also have to be attended, or they would be treated as abandoned property.

The Park Service is, if anything, overcautious. But depend on those who confuse vandalism with liberty to find even these mild measures objectionable.

The American Civil Liberties Union, bless its myopic soul, predictably finds this tightening frivolous, perhaps unconstitutional. "They want to make Lafayette Park look more pretty," said an ACLU spokesman, "(but) we just don't think that is a very weighty concern to justify the infringement of First Amendment rights."

Not a weighty concern? An exercise of rights that blights, all day every day, a public square? How far, one is led to speculate, might libertarian numskullery go?

If someone with a burning message is moved to bedeck the Washington Monument with a huge wraparound banner at the 100-foot level, or hang a sandwich board with anti-nuclear slogans around Mr. Lincoln's neck in the Lincoln Memorial, must the esthetic interests of tens of thousands be dismissed?

Why must the rest of us suffer, in silence, some trashing of the commonweal every time a world-saver with $20 to spend for a signboard and a paint brush goes into action? By long legal usage, even the most essential personal liberties are subject to reasonable "time, place and manner" restriction when their exercise becomes a nuisance or a menace to others.

It is, as Holmes told us, no legitimate exercise of free speech to cry fire falsely in a crowded theater, causing a panic. And not even the silliest judge in the land would uphold your right to ring my doorbell every day at 3 a.m. to deliver your urgent warning against nuclear power.

This is not a plea for banning the right of timely and appropriate petition. It is an argument for measure, and for what might be called the Fifth Freedom: the right to enjoy, unblighted, the graces of the American landscape.

And by the way, while thousands of tourists must seek their first southward glimpse of the White House across a forest of placards, just who is being petitioned for a redress of grievances? Ronald Reagan is at the ranch. Congress is in recess and even when in town does no business near Lafayette Square.

The petitioners and demonstrators should be permitted to do their thing at set times in the park, or in front of the White House, or wherever they wish, then fold up their demonstration sets and move on.

Outrage over the casual spoliation of the American land and cityscape -- of which the trashing of Lafayette Park is part -- is made the keener by visits to European cities. They somehow manage to avoid becoming political gulags without sacrificing their visual grace.

Not so us. Like the clutter erected on the west side of the Executive Office Building, like disfiguring, shoddy, box-like office buildings, steamy parking lots, instant-food strips, daily litter sufficient to make a landfill of the Pacific Ocean bed, the junking of Lafayette Park is of a piece with our national tolerance of ugliness.

And this in the name of liberty? Thomas Jefferson, who had much to say on that subject, was a man of taste who saw that virtues need not be graceless, nor beauty incompatible with liberty. A citizenry that becomes visually brutalized exposes itself to political brutalization as well.