Congressional Democrats and minority groups assailed Monday’s Supreme Court decision upholding Indiana’s photo-ID law as an affront to voting rights, but political realities in the states suggest that the ruling could have relatively limited impact nationwide. Only three states — Indiana, Florida and Georgia — currently require voters to show government-issued photo IDs before stepping into the voting booth. Texas, Oklahoma and Kansas are considering similar requirements, but it’s not clear whether they can adopt them before the November elections. Democratic insiders fear that a number of states, particularly in the Midwest and South, will copy the Indiana law now...