WASHINGTON — Two of the main sponsors of a 2002 law that restricts political donations sued the Federal Election Commission this week, challenging new agency regulations that govern when candidates and independent groups can coordinate their political messages.

The lawsuit, filed by Reps. Christopher Shays, a Republican of Connecticut, and Martin Meehan, a Democrat of Massachusetts, contends that FEC rules effective this week are too lenient. They said the FEC would permit unions, corporations, and wealthy individuals to pay for some political ads and avoid federal restrictions on in-kind contributions.

Though the complaint is highly technical, its outcome could have significant ramifications for congressional and presidential elections by further restricting groups that advocate political causes or by opening new avenues for unchecked political advertising.

The regulations at issue would require labor unions,corporations, or individuals to abide by contribution restrictions and disclosure requirements only if ads they financed appeared within 90 days of a congressional election or 120 days before a presidential election.

The FEC and supporters of the regulation say such a restriction would cover almost all of the independent political advertising that occurs in a campaign cycle. Critics argue that it creates a loophole for advertising that falls outside FEC regulations and ignores the expansion of political campaigns into year-round events.