This decision, in plain English:
The Constitution divides up the power to determine who will be allowed to vote between state governments and Congress. The states have the basic power to decide who is eligible to vote but, so far as state laws are aimed at who gets to vote for President or Congress, the Constitution gives Congress a back-up power to change or even to override those state laws.
The Supreme Court on Monday made a significant effort to try to sort out how to divide up this power, in the context of deciding whether a state may require would-be voters to show proof that they are U.S. citizens — both to register and to actually vote. That proof requirement was challenged by various advocacy groups, because Congress in 1993 had passed a law designed to expand the ranks of voters, and a federal agency acting under that law has specified a form that voters may use to register.
The argument before the Court was that the federal law must control, because Congress had specified that, in filling out a federal form, all would-be voters had to do was to swear they are U.S. citizens, while Arizona went further and required an actual piece of official paper to prove citizenship. The challengers argued that the two approaches cannot co-exist, so the state proof requirement had to yield.
On the one hand, the Supreme Court agreed that, for now, Arizona’s proof requirement must yield to the federal form’s approach — that is, it is enough to register, using that form, if the would-be voter swears that he satisfies the citizenship requirement.
On the other hand, however, the Court also ruled that Arizona can seek permission from federal officials to impose its proof-of-citizenship requirement. If it fails with that request, it can go to court and argue that it has a constitutional right to make proof of citizenship a binding requirement for all voters.
It was the kind of mixed decision that can sometimes baffle lay readers and, in this instance, maybe even lawyers and judges, too, because the two parts of the ruling did not seem to be reconciled easily.