Daily, except for unwelcome trips to court or jail, Concepcion Picciotto has stood her White House vigil since the summer of 1981, protesting the spread of nuclear arms. In those three years, she has been arrested 15 times.

"I believe in the ideals of this country," Picciotto says in her native Spanish accent, vowing to stand fast, within eyeshot of the president of the United States across Pennsylvania Avenue.

She wears a wig the size of a football helmet and a chestful of buttons carrying political slogans. But Picciotto's legal difficulties are not rooted, at least directly, in anyone's suspicion that she is a wide-eyed, bomb-throwing radical.

Instead, Picciotto and other protesters have run afoul of a new thicket of regulations drawn up by the Interior Department, enforced by the U.S. Park Police and recently blessed by the courts, and designed to restrict those who choose the president's doorstep to express their political opinions.

The rules, in effect since May, are expected to get their first major test since winning court approval when demonstrators gather today for a rally called by the Community for Creative Non-Violence.

The regulations leave little to a demonstrator's imagination:

* A demonstrator who is not carrying a placard or sign, for example, may sit to rest on the White House fence ledge. If he rests while holding a placard, he is subject to arrest.

* Signs of up to 20 feet in length and three feet in height may be set up on portions of the White House sidewalk. All must be at least three feet from the fence to guard against concealed explosives.

* Signs constructed of cardboard, posterboard or cloth may be carried by demonstrators. Wooden signs are forbidden because, security experts testified in court this year, they could be used to scale the White House fence.

* In a "central zone" extending 10 yards on either side of the White House fence centerpost along Pennsylvania Avenue, stationary signs are prohibited in order to clear the view for tourists. Demonstrators may be in the "central zone" while carrying signs, if they keep moving.

In a series of recent court cases, federal officials have defended the rules as necessary to protect against acts of terrorism and preserve the beauty of the White House view.

Lawyers for the demonstrators, including the American Civil Liberties Union, continue to complain vigorously that the restrictions are illegal intrusions on First Amendment guarantee of free speech and the right to assemble.

With relatively little fanfare, the onset of the regulations -- prompted, according to the administration, by the threat of a demonstrator to blow up the Washington Monument two years ago -- has turned the White House sidewalk and Lafayette Park into a legal battleground.

"There is no more important place for demonstrators than out in front of the White House," says Jeffrey Pash, a lawyer at the prestigious law firm of Covington and Burling who has represented some indigent protesters in court.

"Whether the First Amendment has been backed into a corner, that might be a little strong. But the regulations are certainly a setback, a very, very severe setback for people like Concepcion."

"Frankly, we were shocked that we had so much trouble getting the rules through the courts," counters Royce C. Lamberth, chief of the U.S. Attorney's office civil division. "The White House is probably one of the most vulnerable living residences of a head of state in the world."

Interior spokeswoman Sandra Alley said Interior received numerous complaints from tourists before the department banished large, permanent protest signs from in front of the mansion to nearby Lafayette Park.

Administration officials say their concerns about presidential security were heightened considerably when, on Dec. 8, 1982, a longtime White House protester, Norman Mayer, backed a truck up to the Washington Monument and threatened to detonate what he said was 1,000 pounds of dynamite. When Mayer tried to drive away, possibly headed for 1600 Pennsylvania Avenue, police shot him to death.

"One thing that was obvious," Lamberth said of the episode, "was, what do we do when the same thing happens at the White House?"

In the view of attorney Pash, security then became "an all-purpose alibi" for a crackdown on demonstrators' activities near the White House. "Whether Mayer caused the new regulations or was more of an excuse is certainly open to question," he says.

As evidence of the administration's attitude, Pash cites a Jan. 13, 1983, memo by then-Interior Secretary James Watt. In the memo, included in a recent court case, Watt asked for a briefing on the demonstrators. "My intention is to prohibit such activities and require that they take place on the Ellipse," well south of the White House grounds, Watt said. His sweeping idea never took effect, but Interior moved to implement its stricter regulations soon afterward.

Earlier this year, U.S. District Judge William B. Bryant threw out the rules, branding them "totally ineffective" and "demonstrably too vague." Bryant's ruling, however, was stayed by the U.S. Court of Appeals while the government's appeal was pending.

Last Friday, the U.S. Court of Appeals, in White House Vigil for the ERA Committee v. William P. Clark, upheld the regulations for both security and esthetics reasons.

In the face of such pronouncements, Picciotto says she tries to toe the Interior Department's line. "I just moved here," she says, indicating a patch of sidewalk in Lafayette Park where demonstrators have been told to stand during the reseeding of the park lawns.

"I think I'm all right here. But I'm not sure."