The impact would likely be smaller if the court only strikes down part of the ACA. The justices are expected to decide Thursday whether the law's individual mandate, which requires most people to buy insurance or pay a penalty, is constitutional. If it's not, they can strike the entire law or just part of it.
Although losing the mandate would probably lead to a drop in coverage, the law's Medicaid expansion would likely remain intact.
The 26 states that challenged the mandate also argued that the Medicaid expansion is unconstitutional, saying it is "coercive" and takes away states' legal right to opt out of Medicaid. But the justices are not expected to side with the states on that count, and would likely invalidate the Medicaid expansion only if they strike down the entire law.
According to Avalere, several states have already expanded their Medicaid eligibility because of the ACA. The law also barred states from cutting their existing eligibility levels until 2014, when the federally funded expansion kicks in. If the entire healthcare law falls, those safeguards would no longer be in place.