ATLANTA — Georgia's electorate is changing fast, state officials said Tuesday, underscoring a shift that Democrats see as critical to making gains in a state dominated by Republicans. The total number of people registered to vote in Georgia has dropped by nearly 40,000 since the last election. But of the 183,000 voters added to the rolls since last March, only a third identified themselves as white, a key statistic in the race for an open Senate seat and the national battle for control of the chamber in the final two years of President Barack Obama's time in office.