Tough week for Obama.

The US Supreme Court has agreed to hear a conservative challenge to health insurance subsidies, which are a key part of his signature health care reform package.

It’s considered perhaps the biggest current threat to Obamacare. It hinges on the question of whether the federal government can give financial aid to people who buy insurance at HealthCare.gov. Those subsidies go to 86% of exchange customers.

It wouldn’t be decided until around May 2016, just as Obama’s term was winding down.

Obamacare