The Obama administration is planning to use the government’s enormous buying power to prod private companies to improve wages and benefits for millions of workers, according to White House officials and several interest groups briefed on the plan. By altering how it awards $500 billion in contracts each year, the government would disqualify more companies with labor, environmental or other violations and give an edge to companies that offer better levels of pay, health coverage, pensions and other benefits, the officials said. Because nearly one in four workers is employed by companies that have contracts with the federal government, administration...