WASHINGTON — The Supreme Court has agreed to hear a new challenge to President Barack Obama’s health care law.

The justices today said they will decide whether the law authorizes subsidies that help millions of low- and middle-income people afford their health insurance premiums.

A federal appeals court upheld Internal Revenue Service regulations that allow health-insurance tax credits under the Affordable Care Act for consumers in all 50 states. Opponents argue that most of the subsidies are illegal.

The long-running political and legal campaign to overturn or limit the 2010 health overhaul will be making its second appearance at the Supreme Court.

The justices upheld the heart of the law in a 5-4 decision in 2012 in which Chief Justice John Roberts provided the decisive vote.