In 2012, the Supreme Court ruled that a cornerstone of the Affordable Care Act — its expansion of Medicaid to low-income people around the country — must be optional for states. But what if it had ruled differently?

More than three million people, many of them across the South, would now have health insurance through Medicaid, according to an Upshot analysis of data from Enroll America and Civis Analytics. The uninsured rate would be two percentage points lower.

Today, the odds of having health insurance are much lower for people living in Tennessee than in neighboring Kentucky, for example, and lower in Texas than in Arkansas. Sharp differences are seen outside the South, too. Maine, which didn’t expand Medicaid, has many more residents without insurance than neighboring New Hampshire. In a hypothetical world with a different Supreme Court ruling, those differences would be smoothed out.

And that was the idea behind the Affordable Care Act. Before the law passed in 2010, the country had a highly regional approach to health policy and widely disparate results in both health insurance status and measures of public health. One of the main goals of the law was to provide some national standards and reduce those inequities by using federal dollars to buy coverage for low-income people in every state.

In many ways, as the Enroll/Civis data highlights, the law has succeeded in bringing health insurance to the disadvantaged populations who have historically lacked it. Its model shows that the biggest beneficiaries of the law have been the very groups that tend to suffer in today’s economy: blacks and Hispanics; young adults; people living in rural areas; women; and those earning the lowest incomes. (For a detailed look at who was covered by the law this year, read our article on the most salient trends.)

But the uneven Medicaid expansion, largely a result of Republican politicians’ dislike of the program and their concern that their states might get stuck with the costs, has limited the law’s ability to cover poor Americans living in many of the poorest states in the country. Currently, 26 states and the District of Columbia have expanded their programs. In the country’s two poorest states, Mississippi and Louisiana, more than 15 percent of the population between 18 and 65 remains uninsured in 2014, the data show.

Consider New Mexico and Texas, two states with among the highest rates of the uninsured in 2013. The gray arrow shows how much the uninsured rate has declined in the last year, according to the Enroll/Civis data. In reality, New Mexico expanded its program while Texas did not. But the green arrow shows our estimate for how much more Texas’ rate might have declined if it had expanded, too.

New Mexico expanded Medicaid, and its uninsurance rate dropped to 11.5 percent:

Texas did not, but the policy would have reduced its uninsured population significantly.

Comparing Texas and New Mexico

Here’s a similar chart showing how an across-the-board expansion might have looked in every state.

Chart of All States

Nearly all the states at the bottom of the list were those that did not expand. That’s because many started with high percentages of uninsured people and made smaller reductions than states that did expand. But if every state had expanded, many states now near the bottom of the list would have moved up.

Medicaid has some disadvantages when compared with employer-based insurance; in many states, people covered under the plan can have trouble finding doctors who will treat them. But surveys have shown that the program is popular among the people who use it. And it can protect low-income people from financial calamity if they have an accident or serious illness that brings large hospital bills.

Our what-if map is, of course, a kind of thought experiment; it shouldn’t be read as a perfect prediction. The Enroll/Civis model is devised to estimate who had health insurance in 2013 and who has it now, based on interviews with thousands of adults along with commercial and government data. (Read more about the estimates and how they were made here.) At The Upshot’s request, Civis reran its calculations assuming every state had expanded Medicaid. Several factors beyond a mere decision to expand would have influenced each state’s enrollment in Medicaid, including how easy officials made it to sign up and how hard they worked to get the word out. Those sorts of state differences aren’t captured by this type of estimation, which is based on data on enrollment in the states that did expand. But though our numbers are estimates, they still give us an approximate picture of what the country might have looked like today without the court decision.

Because some states that have not expanded Medicaid may do so later, the data also gives us a glimpse of the possible future in some states.

Take Pennsylvania: Its governor, a Republican, recently reached an agreement with the federal government, and Medicaid coverage will be expanded in January. In the current situation, Pennsylvania stands out as a rare mid-Atlantic state with a modest rise in the percentage of residents with health insurance. Our hypothetical map shows what the situation may be there next year.

Another state worth looking at is Maine. In that state, the legislature voted five times to expand the program, but Gov. Paul LePage vetoed the bill each time. Mr. LePage is up for re-election Tuesday, in what looks to be a tight race. If beaten by his Democratic challenger, Mike Michaud, then Maine, too, might begin to look more as it does in our hypothetical map.