Legal challenges to state voter regulations are still drawing political and judicial scrutiny four months after the November election.
This was evident in a divisive oral argument on Monday at the Supreme Court over whether states can enhance a federal voter registration law with their own requirements.
The justices appeared at odds over Arizona's voter-approved Proposition 200 and its insistence on proof of citizenship before registering.
The state calls it a "sensible precaution" to prevent voter fraud. Civil rights groups counter that it adds an unconstitutional and burdensome layer of paperwork for tens of thousands of citizens.
"If I see the purpose of the [federal law] is to simplify registration, how is Arizona's provision consistent with that objective and purpose," said Justice Sonia Sotomayor, given "that many people don't have the documents that Arizona requires?"
But Justice Antonin Scalia spoke for several of his conservative colleagues, asking, "What is -- in the words of the [federal] statute -- 'necessary to enable' for the appropriate state election official to assess the eligibility of the applicant? It's clear that the statute intends the states to be able to do that."
The appeal is a classic federalism issue on the often delicate line between conflict and cooperation between state and federal governments over enforcing voting procedures.
The current fight has produced a range of states, lawmakers, and advocacy groups on both sides of the issue. The Obama administration opposes the law.
Retired justice Sandra Day O'Connor, an Arizona native, was among those attending the hour-long arguments.