According to a nationwide Gallup Poll, the Affordable Care Act (ACA) has steadily reduced the number of people without insurance across the United States. The group conducted phone interviews (landline and cell) in 2013 and 2014, as part of the Gallup-Healthways Well-Being Index, with a random sample of 178,072 adults in 2013 and 176,702 adults in 2014. The survey found that the nationwide rate of uninsured adults declined from 17.3 percent in 2013 to 13.8 percent in 2014.

The lowest uninsured rates continue to be primarily in the Northeast and upper Midwest. Massachusetts, whose 2006 health care coverage expansion became the model for the national law, had the lowest rate at 4.6 percent. The highest uninsured rates are in the South and West. For the seventh consecutive year, Texas has the worst rate in the country, with nearly a quarter of its adults uninsured.

California, historically the state with the highest uninsured rate, recorded one of the fastest declines. The share of adults without coverage in the state fell from 21.6 to 15.3 percent.

States that have implemented a Medicaid expansion and state health exchanges are seeing a substantially larger drop in the uninsured rate than states that did not take both of these actions.

The survey results came just a week before the U.S. Supreme Court hears oral arguments in a lawsuit challenging ACA’s core structure. On March 4, the court will hear King v. Burwell, a case that questions whether premium subsidies can be provided under the law to individuals purchasing health insurance coverage on exchanges run by the federal government.

The lawsuit has the potential to affect 36 states that use the federal health care exchange, but would not change the subsidies in states like California that run their own exchanges.