Some Democrats and their advocates in the press believe Obamacare, a year into implementation, is no longer much of a factor in the midterm elections. But no one has told Republican candidates, who are still pounding away at the Affordable Care Act on the stump. And no one has told voters, especially those in states with closely contested Senate races, who regularly place it among the top issues of the campaign. In Arkansas, Republican challenger Tom Cotton is pulling ahead of incumbent Democratic Sen. Mark Pryor partly on the strength of a relentless focus on Obamacare. Cotton's newest ad attacks...