Federal regulators who must enforce a sweeping new campaign finance law and those telling them how they should do it are finding the devil is truly in the details.

At a Federal Election Commission hearing Tuesday on its proposed rule to implement a cornerstone of the new law, much of the discussion focused on how the FEC should define one word: agent.

At issue is how and to whom the FEC should apply the law's ban on the raising and spending of "soft money" -- unlimited contributions from labor unions, corporations and others -- by federal politicians for federal election activity and by national party committees.

Campaign-finance watchdogs who back the new law told the FEC it should take a broad view of it while trying to write straightforward rules. Critics of the law say the commission's rules should carry "bright-line" tests spelling out what political players can and can't do.

"Clarity should never be a cover for going against the intent of Congress," said Larry Noble, executive director of the Center for Responsive Politics and a former FEC attorney. "The FEC must avoid these calls to eviscerate the law right out of the box."

The law's sponsors, including Sens. John McCain, R-Ariz., and Russ Feingold, D-Wis., say the FEC's rules should make clear that national party committees and federal candidates and officeholders cannot avoid the ban by using staff or other intermediaries to raise soft money.

Several interest groups and political parties are urging the FEC to take a narrow view of the law, arguing a far-reaching approach would violate Congress' wishes.

AFL-CIO attorney Larry Gold said the ban on raising soft money should apply only to those who have actual express oral or written authority to raise such money on behalf of a national political party or federal politician.

Many union members involved in AFL-CIO grass-roots political activities such as volunteer get-out-the-vote drives also serve on Democratic party committees, and an overly broad ban could unfairly ensnare them, Gold said.

He told the FEC it should be faithful to the law and give no special weight to the sponsors' comments, particularly of what they seek is not part of the legislative history.

The sponsors do not "have some kind of continuing role of interpreting, advising, instructing or requiring the commission to decide anything, let alone the courts," Gold said.

Ki Hong, an attorney for the NAACP National Voter Fund, said he was concerned about how the FEC would handle tax-exempt groups like the fund that have received soft money from political party committees. It should only consider money they accept after the law takes effect, he said.

He also asked the commission to let national parties contribute any remaining soft money to nonprofit organizations like the voter fund when the new law takes effect after the November election.

The FEC's hearing was to continue Wednesday with testimony from political party committees including the Republican National Committee and Democratic National Committee.

The commission plans to adopt soft money rules by the end of June. It will then begin work implementing other parts of the overhaul. The law faces several court cases over its constitutionality.