Some Republican-led states say they they will block Justice Department’s election monitors from going inside polling places on Election Day, pushing back on federal authorities’ decades-long practice of watching for violations of federal voting laws.
Federal law supercedes state law. That doesn’t mean federal officials can go where they want.
IIRC, the issue is that the Voting Rights Act gave DoJ the power to observe elections, but the SCOTUS partially nuked the VRA. So now it’s not clear whether the DoJ still has that power, and the usual suspects are taking advantage of this ambiguity. Possibly hoping to give the SCOTUS a chance to nuke more of the VRA.