LAS VEGAS — Hillary Rodham Clinton and Sen. Bernie Sanders clashed over U.S. involvement in the Middle East, gun control and economic policy in the first Democratic presidential debate Tuesday night, vigorously outlining competing visions for a party seeking to keep the White House for a third straight term. Yet in a moment of political unity — and levity — Sanders leapt to Clinton's defense on the issue of her controversial email practices as secretary of state. "The American people are sick and tired are hearing about your damn emails," Sanders exclaimed as the crowd in Las Vegas roared with applause.