The AFL-CIO is asking for changes to the Affordable Care Act, President Barack Obama's landmark health-care overhaul, potentially opening fissures between the White House and one of its staunch political allies. The largest U.S. labor federation approved a resolution yesterday urging amendments to the law in a voice vote on the final day of its quadrennial convention in Los Angeles. The action by the AFL-CIO, which endorsed the law when it passed in 2010, adds to the political challenges facing the Obama administration as it implements the law. Republicans have vowed to block the union-backed changes, making it unlikely they could get through Congress.