Critics of the new healthcare law are mounting a challenge to President Obama’s interpretation of another important provision, health insurance exchanges, under which the federal government will subsidize health insurance for millions of low- and middle-income people. At issue is whether the subsidies will be available in exchanges set up and run by the federal government in states that fail or refuse to establish their own exchanges. Critics say the law allows subsidies only for people who obtain coverage through state-run exchanges. The White House says the law can be read to allow subsidies for people who get coverage in federal exchanges as well.