A coalition of 13 national nonprofit organizations yesterday sued the Office of Personnel Management, seeking to block its requirement that charities in the Combined Federal Campaign screen employees for connections to terrorist organizations.

In their suit, the American Civil Liberties Union, the NAACP, Amnesty International and 10 other nonprofits said that the OPM did not follow proper procedures in instituting the policy and that it is unconstitutional.

"If the government were to ask us to do something to ensure that we were not inadvertently supporting terrorists that was clear . . . and definitive . . . we would not hesitate to comply," Hilary O. Shelton, director of the NAACP's Washington bureau, said at a news conference yesterday. "But that has not happened here."

The suit is the culmination of a months-long dispute between some CFC participants and the OPM over a little-noticed regulation the agency issued last year. Under the rule, the 10,000 nonprofits that participate in the CFC are required to sign a pledge that they do not "knowingly employ" suspected terrorists. The charities are required to consult lists maintained by the government.

But nonprofits have protested that the terrorist "watch" lists are riddled with errors and that the rule is too vague and requires charities to police the association of their employees.

"It's not our job," said Audrey R. Alvarado, executive director of the National Council of Nonprofit Associations, whose members represent 17,000 nonprofits. The council is supporting the suit, but is not a participant in it. "There are appropriate entities that have far more resources to do that kind of work," Alvarado said.

The suit, filed in U.S. District Court, accuses the OPM and Mara T. Patermaster, the OPM's director of CFC operations, of failing to publish the new rule in the Federal Register to allow public comment. It also says the rule violates nonprofits' freedom of speech.

Nationwide, about 1.3 million federal employees pledge funds through the CFC. Their donations are deducted from their paychecks throughout the year and forwarded to their designated charities. Charities must apply to be included in the CFC. Last year, the drive raised $248 million.

Several nonprofits participating in the suit have withdrawn from the CFC to protest the rule, including the ACLU, which received $500,000 from the campaign last year; Amnesty International, which got $250,000; and the Brennan Center for Justice, recipient of $2,000 a year.

The NAACP, which received $170,000 from the CFC drive last year, said it has not decided whether to withdraw.

The OPM referred calls for comment to the Justice Department, where a spokesman declined to comment, saying he had not seen the suit.

But the OPM has said it is required to impose the screening under an executive order signed by President Bush shortly after the Sept. 11, 2001, attacks. The order forbids U.S. citizens to have financial ties with suspected terrorists or terrorist organizations.