More than 800,000 Floridians would see their monthly insurance premiums rise, from an average of around $70 to an average of around $350, or roughly a factor of five. More than 600,000 people in Texas, about 325,000 in North Carolina, and another 275,000 in Georgia would see insurance premiums soar by similar amounts.

Nationwide, more than 4 million people living in 37 states would be in situations like these. Most would have no way to pay the higher bills, forcing them to drop insurance coverage altogether. Their sudden absence would destabilize insurance markets in those states, giving carriers reason to raise premiums by additional amounts or to flee the states altogether—which would, in turn, lead more people to give up insurance.

The underlying premiums for all people buying insurance on their own in these states would rise by an average of 43 percent, while the number of Americans without insurance would eventually be 7 million higher than it would be otherwise.

Based on some crude extrapolations from data and studies by leading researchers, these are the things likely to happen if the Supreme Court rules in favor of a lawsuit challenging part of the Affordable Care Act. The Court agreed to hear the case, King v. Burwell, on Friday. The central issue is the federal government’s ability to distribute tax credits, which allow people buying coverage through Obamacare's new marketplaces to pay less than full price—in some cases, a lot less. They are available, on a sliding scale, to people with incomes between 100 and 400 percent of the poverty line, or about $23,850 and $95,400 for a family of four. They are worth hundreds and, in many cases, thousands of dollars a year. Overall, approximately 6 million Americans are getting these tax credits.

But some of these people live in states like California, Kentucky, and Maryland, where officials decided to build and operate their own marketplaces. The majority live in states like Florida, Pennsylvania, and Texas, where officials handed that work off to the federal government. The section of the law authorizing the government to distribute tax credits doesn’t mention these federally run marketplaces explicitly; it mentions only marketplaces “established by a state.” The lawsuit claims this omission was intentional and that, as a result, no tax credits are supposed to be available in states that don’t run their own marketplaces.