At the Democrats' first debate in the presidential season, candidates jockeyed for bragging rights over who had the lowest rating from the National Rifle Association. The return of the gun debate comes in the first White House contest since the December 2012 shootings in Newtown, Connecticut, that killed 20 children and six educators at an elementary school, and a string of mass shootings after that. Democrats say support for new gun laws is broader now and the politics of the issue have shifted enough to make the push for tougher measures a political winner, even if there remains almost no chance for success in Congress. Hillary Rodham Clinton has kept up a drumbeat for weeks, using the issue to try to drive a wedge between liberal Democrats and her top rival, Vermont Sen.