Authors of the new campaign finance law that bans so-called soft money warned the Federal Election Commission this week that its proposed rules to enforce the law would create loopholes and thwart their mission to keep elections clean.

Sens. John McCain, R-Ariz., and Russ Feingold, R-Wis., and Reps. Christopher Shays, R-Conn., and Martin Meehan, D-Mass., told the FEC in written comments that the commission's rules must be true to the intent of the new campaign finance law.

"Throughout our fight for this reform law, skeptics regularly told us what we were doing would not reduce the impact big money was having on politics and legislation," they wrote Wednesday. "We therefore worked hard to craft a tight and effective ban, and we ask the commission to be true to that goal as it crafts the final rules."

They said that the proposed FEC definitions for terms in the law, which would spell out to contributors what they can and cannot do, would "instead create loopholes though which the existing system could continue to operate."

For example, in its proposal the FEC defines "agent," a designation that will give an individual the express authority to act for a candidate or party committee. But the lawmakers said the definition is too vague and could open the way for candidates to use staff and other intermediaries to raise and spend soft money.

"In the political world, many individuals have titles or positions that lead the general public or potential donors to believe that they are acting on behalf of candidates or parties," the law's sponsors wrote. "When that is the case, the candidate or party must be held accountable for the actions of those individuals."

Lawmakers want to make sure that money from unions, corporations and others don't spoil the election system by buying influence. McCain and others say rules must be made to determine in which type of get-out-the-vote activities those interest groups can participate.

The law, which won't go into effect until after the election, still faces several court challenges over its constitutionality, includes restrictions on election-time political advertising.

The FEC is scheduled to meet next week to discuss what soft money rules to adopt and what advertising to limit.

The Associated Press contributed to this report.