Haley Barbour, the Republican governor of Mississippi, has questioned the legality of tens of thousands of new voter applications in his state, raising the prospect that new voters might be challenged on election day.

Mr Barbour told the Financial Times that “not all” newly registered voters in Mississippi were legal because of rules that require voters who register by mail to include a photocopy of a federally accepted identification in their application, or bring identification to the polls. “I suspect some of those people won’t be able to do that,” he said.