WASHINGTON — Field workers for President Obama’s campaign fanned out across the country over the weekend in an effort to confront a barrage of new voter identification laws that strategists say threaten the campaign’s hopes for registering new voters ahead of the November election. Many of the laws in question — including the ones in Florida and Wisconsin — are the subject of legal challenges by Democratic groups who say they are part of a partisan, Republican effort to dampen the turnout of voters, particularly members of minority groups, for Mr. Obama and his party. But senior aides to Mr....