Social Security has always been an issue that united the Democrats like no other. But suddenly, the most successful and popular government program in history is a subject of fractious debate in their party's presidential primary. Barack Obama suggests that Hillary Clinton is refusing to engage in "a real, honest conversation" about the challenges that lie ahead for the program. And Clinton is accusing Obama of buying into "Republican scare tactics."
At issue: Obama's proposal to increase the taxes that wealthier Americans pay into the system. Currently, workers are taxed on only the first $97,500 of what they earn, a limit...