The nation's largest health insurer warned Thursday that it may pull out of the Obamacare exchanges after 2016 – forcing more than a half million people to find other coverage – after low enrollment and high usage cost the company millions of dollars. The possible move by UnitedHealth Group raises new questions about the viability of President Obama's signature health law and follows the departure of more than half of the non-profit insurance cooperatives this year. If UnitedHealth drops out, consumers would lose one of the lowest-cost plans available in much of the country, and some wonder how smaller insurers could fill the void. “If they can’t make money on the exchanges, it seems it would be hard for anyone,” said Katherine Hempstead, who heads the insurance coverage team at the Robert Wood Johnson Foundation.