More than that, he said, if they find evidence of criminal misconduct, they have to turn it over to the Justice Department.
"If they come up with a [criminal] violation, they are obligated to refer it to the Justice Department," he added.
Ensign had already announced he would not seek re-election, but a month and a half after the Senate Ethics Committee named a special prosecutor to his case, Ensign decided it was time to go -- and quickly. His resignation is effective May 4.
Watchdogs immediately worried that the ethics panel would let Ensign off the hook completely, but the panel quickly quashed that concern by issuing a rare statement signaling it would continue its investigation.