With the Supreme Court next week taking up oral arguments in a lawsuit challenging Obamacare's core structure, a new Gallup poll is underscoring the lawsuit's high stakes.

The nation's uninsured rate dropped from 17.3 percent to 13.8 percent in 2014, according to the Gallup-Healthways Well-Being Index. That marks the lowest rate in the seven years the firm has been tracking the figure.

States that fully embraced the law — setting up their own health insurance marketplaces and expanding Medicaid — on average saw their uninsured rates drop 4.8 percentage points last year, compared to the 2.7 percentage point drop recorded by states that didn't opt for both major pieces of the law's coverage expansion.

No state saw a significant increase in their uninsured rate last year, the first year of expanded coverage under the Affordable Care Act. But southern states, which include the staunchest opponents of the Affordable Care Act, continue to rank among those with the highest rates of the uninsured, as the following map shows.

Still, a pair of Southern states where Democratic governors pushed implementation of the ACA recorded the greatest insurance gains in 2014. The uninsured rate in Arkansas, which used a pioneering Medicaid expansion plan, dropped from 22.5 percent to 11.4 percent last year. In Kentucky, where Gov. Steve Beshear took executive action to create an exchange and expand Medicaid, the uninsured rate dropped from 20.4 percent to 9.8 percent.

The uninsured rate in Massachusetts, where its 2006 health-care law served as the basis for the ACA, continues to be the lowest in the nation, at 4.6 percent. Texas, which had one of the highest signup totals on HealthCare.gov in 2015, again had the highest uninsured rate last year, though it dropped from 27 percent to 24.4 percent.

The Supreme Court next week will consider a challenge to the subsidies that make health insurance sold through the exchanges more affordable. The challengers contend that the text of the ACA intentionally providedinsurance subsidies on the condition that states setup their own exchanges, something that just 16 states and the District of the Columbia have done. The law's authors and ACA supporters arguethe text makes clear that all exchanges, regardless of whether they're state- or federal-run, can provide financial assistance.

If the challengers are successful, anywhere between 8 million and 10 million people could become uninsured, according to estimates. That's unless Washington and the states can come together and figure out how to save the subsides for the many Americans now dependent on them.