Oregon, Rhode Island and Washington also have exceeded a 10-percentage-point drop since 2013.

Each of those states has chosen both to expand Medicaid and set up their own state exchange or partnership under the Patient Protection and Affordable Care Act — two keys measures that have proved to significantly reduce the number of uninsured compared to states that didn't take these steps.

The uninsured rate dropped an average of 7.1 points in the 22 states that implemented both of these measures by Dec. 31, 2014, compared with a 5.3-point drop across the 28 states that had implemented only one or neither of these actions, Gallup said.

"While some additional progress can be made, therefore, in the reduction of the uninsured rate via further Medicaid expansion, this mechanism for reduction has likely reached most of its potential unless additional states choose to implement it," Gallup reported. "As such, the marketplace exchanges that enable people to select and purchase their own plan directly from insurers will likely be the primary means by which the national uninsured rate would be reduced in the immediate future."

There are now seven states with uninsured rates that are at or below 5 percent, Gallup found: Rhode Island, Massachusetts, Vermont, Minnesota, Iowa, Connecticut and Hawaii. No state has reported a statistically significant increase in the percentage of uninsured thus far in 2015 compared with 2013.

The polling firm — as well as other consulting firms — has been tracking the nation's continuing falling uninsured rate over the last several years. Other research by the Urban Institute in April mirrors the Gallup finding that Medicaid expansion states are leading the decline in the uninsured rate.

Thirty states so far have expanded Medicaid under the law.