Who are the feds going to arrest? The Sheriff's? They're elected.

They can't arrest them.

The feds will pull their federal funding and open up the Sheriff's to lawsuits that will cripple their offices. With no money, they can't employ deputies. No deputies means no law enforcing. No enforcing and feds likely encouraging shit bags to go to the county to commit crime means the people will turn on the sheriff and recall/vote him out where a more 'pliable' sheriff will take office.