(Bloomberg) — A decision halting the credits might unravel the Affordable Care Act, making other core provisions ineffective and potentially causing the market for individual insurance policies to collapse in much of the country. Hospitals could be left with billions of dollars in unpaid bills.

"This would be the greatest instance of judicial overreach in the modern history of the court," Senator Chris Murphy, a Democrat from Connecticut, told reporters on Tuesday. The court will rule by the end of June.

A decision against the administration would put pressure on the states, most of them Republican-controlled, that have refused to set up their own exchanges, as the insurance marketplaces are known. Residents of those states would face the prospect of losing tax credits. Congress could step in, though it is riven by opposition to the health-care law. The Obama administration says it can do little on its own.