President Obama’s signature healthcare law is back before the U.S. Supreme Court today, in a case that could determine whether millions of low-income people lose their health insurance. Right-wing opponents of Obamacare are waging a technical argument against specific language in the law, which concerns tax subsidies to help people afford insurance. The law’s challengers claim the language does not allow people to receive subsidies if they get their insurance through a federal exchange instead of one established by the state. If the Supreme Court sides with the challengers, more than nine million people in the largely Republican states that have rejected state exchanges could lose their healthcare. The case marks the third challenge to Obamacare heard by the Supreme Court. In 2012, the court narrowly upheld the individual mandate at the law’s core. Last year, justices sided with religious corporations in the Hobby Lobby case, which weakened access to birth control.