Under Supreme Court precedent, a state doesn't have the power to block the enforcement of federal law inside its borders, said Joseph Thai, a constitutional law professor at the University of Oklahoma who served as law clerk to Supreme Court Justices John Paul Stevens and Byron White.

NPR: States' Rights Bills Surge In GOP States

In Weston, though the town clerk was happy to give Mr Perry and Mr Evans their civil licence, four of the town's five justices of the peace are unwilling to solemnise a civil union.

ECONOMIST: Homosexual unions