The FEC needs four of six members to enforce the law. It now has three.

The resignation of Vice Chairman Matthew S. Petersen, announced on Monday and scheduled for the end of August, will effectively freeze the F.E.C.’s governance, leaving it one person short of a quorum and thus unable to take on some of its most basic actions, including holding board meetings, starting audits, making new rules and levying fines for campaign finance violations.

“Voters should be extremely concerned,” said Ann M. Ravel, a Democrat and former F.E.C. chairwoman who stepped down in 2017 and who has not been replaced. “If you do not have the ability to do any kind of enforcement, then there isn’t any kind of respect for the law.”

“It could end up being the Wild West,” Ms. Ravel added.

Four F.E.C. commissioners are necessary to meet, and there will be only three left on what should be a six-person governing board. The terms of all three commissioners who are still on the board have expired, though the law allows them to remain.