Washington — A growing divide over Social Security splits the two leading contenders for the Republican presidential nomination, and the differences between Texas Gov. Rick Perry and former Massachusetts Gov. Mitt Romney foreshadow a tricky political dance with older voters. Romney has seized on what he perceives as Perry’s vulnerability on a program that seniors hold dear, Democrats venerate as sacrosanct and Perry has labeled a “Ponzi scheme.” That sets up a battle for older voters in Iowa, retirees in Florida and the Sun Belt, and Baby Boomers everywhere worried about their own recession-scarred retirement plans. “If we nominate someone...