Predictably, Democrats went after Mitt Romney's new running mate immediately, describing Paul Ryan as a "certifiable right-wing ideologue" whose views are "extreme" and "radical." They focused on Medicare, warning that Republicans "would end Medicare as we know it," making it "a voucher system" that costs seniors "thousands of dollars in health care costs."

Some Republican hand-wringers moaned. They failed to consider that Democrats were going to level these charges no matter whom Mr. Romney picked as his running mate. And...