The U.S. Office of Special Counsel has warned federal employees working at national parks, monuments and other government facilities that they may be breaking the law if they allow the presidential contenders to campaign at their work spaces.

The OSC said in an Aug. 9 advisory that government employees are not permitted to authorize candidates running in partisan elections to use "federal facilities" for speeches, photo opportunities, news conferences or any other type of campaign event. That, it said, would violate the Hatch Act, a decades-old law that limits federal employees' political activities while on the job. Those found in violation of the law could be suspended or fired.

The agency has jurisdiction over the Hatch Act.

"The Hatch Act should be considered carefully when handling a candidate's request to visit or use a federal building," the advisory said. "We strongly encourage all federal agencies receiving such requests to contact OSC prior to granting such a request."

The OSC said candidates who hold elected office are allowed to visit the sites on official business. But it warned employees who authorize such events that it was their responsibility to "ensure" that the candidates do not engage in any sort of electioneering.

"During this busy campaign season, we want federal employees to be scrupulous about the restrictions concerning the use of their official position or federal property for campaign-related events or activities," Special Counsel Scott J. Bloch said in a recent statement. "The federal workplace must be free of undue influence and is no place for partisan campaign activity."

But critics said the memo was a new interpretation of the law, designed to prevent -- or least make it more difficult for -- Democratic presidential nominee John F. Kerry to stage photo opportunities, for example, at such venues as the Grand Canyon.

"When the president goes to visit a facility or receive a briefing and gives a speech about how wonderful feds are doing or how we're winning the war on terrorism, that's an expression of official duties," said Paul C. Light, a government professor at New York University. "But when a presidential candidate from the other party goes out and does that, that's a campaign event.

Phil Singer, a spokesman for the Kerry campaign, said it has not had any problems visiting federal sites. But he denounced the advisory as hypocritical.

"It's a little ironic that the Bush White House is suddenly growing concerned about the Hatch Act, considering that it has spent the better part of this campaign figuring out new and innovative ways to use official resources to advance the Bush reelection effort," Singer said.

Jeff Ruch, the executive director of the advocacy group Public Employees for Environmental Responsibility, complained that the agency's long-standing prohibitions on federal employees wearing or distributing campaign-related paraphernalia while on the job are so vaguely worded in the advisory that it is creating confusion among workers as to what they can and cannot do. He said some employees are wondering if they must remove campaign bumper stickers from their personal vehicles if they park in government lots.

An OSC official, who would not comment except anonymously, defended the advisory, saying the policy was neither new nor biased toward any particular candidate.

"Any U.S. senator or U.S. congressman has national policy scope," he said. "There's no disparity."

He also said the policy will not necessarily prevent any candidates from campaigning on federal properties such as the Grand Canyon. He said the details will vary by site, but in general the office's restrictions apply only to buildings where federal employees work and any other areas they frequently use.

An anonymous aide said restrictions don't bar candidates, such as John Kerry last month, from visiting the Grand Canyon.