The raid comes just four weeks before early voting begins in Texas. Cuellar faces a former staffer, Jessica Cisneros, in a race that's expected to be close.

The FBI does not openly investigate politicians before an election without intense deliberation. The director often limits overt investigative activity involving elected officials 60 or 90 days before a contested race, Bash said.

"The decision to do this at this time... this is either an incredibly serious allegation or a very fast-moving investigation," Patrick said.