The Supreme Court on Friday blocked the Education Department from canceling up to $20,000 of student debt owed by tens of millions of Americans, thwarting a major domestic priority of President Joe Biden as he campaigns for reelection.

In a 6-3 decision, the court’s conservative majority ruled that Biden’s effort to erase roughly $400 billion of student debt was an illegal use of executive power. The court rejected the Biden administration’s argument that it could enact mass debt forgiveness by using emergency powers tied to the Covid-19 pandemic.

The ruling immediately upends debt relief the Education Department approved last fall for 16 million borrowers and the pending applications for millions of additional borrowers.

And the defeat for the Biden administration will further increase progressive pressure on the White House to come up with an alternative plan to provide relief to student borrowers using a different legal authority.