When it comes to health care coverage, America is becoming a land of geographically based haves and have-nots.

States with the lowest uninsured rates are clustered in the Northeast and upper Midwest, while those with the highest rates of uninsured Americans are mostly in Southern states such as Georgia and Louisiana, according to a new study from Gallup. One reason is that many Southern states opted out of expanding Medicaid coverage under Obamacare.

On the other hand, the states that both expanded Medicaid and set up their own state exchanges saw a bigger drop in uninsured residents than did states that opted out, the study found. In the 21 states that adopted both measures, the uninsured rate fell 4.8 points, while states that shunned one or both options saw a 2.7 point decline.

"Consequently, the gap in uninsured rates that existed between these two groups in 2013 nearly doubled in 2014," Gallup said in its report.

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With almost a quarter of its residents lacking health insurance, Texas has the highest uninsured rate among the 50 states, although that's still lower than state's 27 percent uninsured rate in 2013, Gallup found.

The state with the lowest uninsured rate is Massachusetts, where only 4.6 percent of its residents lack insurance.

"Massachusetts' plan is based on the 'Romneycare' model, from which certain aspects of the Affordable Care Act were drawn," Gallup said. "The Massachusetts healthcare law, originally passed in 2006, has undergone modifications to accommodate details of the federal law, but maintains its core element mandating that nearly all residents must obtain a minimum level of insurance coverage."

Most Southern states have poverty rates that are higher than the U.S. average, according to the Journal of American Medical Association. About 28 percent of residents in Louisiana and Mississippi live in poverty, compared with 21 percent for the U.S. on average. About 4 million poor, uninsured adults in Southern states that didn't expand Medicaid fall into a coverage gap, which also disproportionately affects people of color, JAMA said last year.

States that have rejected Medicaid expansion, such as Texas, cited concerns about long-term costs because states are on the hook for paying a portion of those Medicaid expansion costs starting in 2017.

To be sure, some Southern states opted to expand Medicaid, and Gallup cited two of those as having the sharpest reductions in uninsured rates. In 2013, 22.5 percent of Arkansas residents were uninsured, but that dropped to 11.4 percent in 2014. Kentucky saw the second-biggest decline as its uninsured rate fell from 20.4 percent in 2013 to just under 10 percent last year.

While Obamacare remains unpopular with a majority of Americans, it has had a positive impact in reducing the ranks of the uninsured, Gallup noted. Across the country, the uninsured rate slipped 3.5 percent to 13.8 percent in 2014. That's the lowest rate in the seven years Gallup has been tracking it.