Care about Obamacare? Then you should really care about Arkansas.

Three weeks ago, when Arkansas governor Mike Beebe was in Washington for the National Governors Association meeting, he made a trip to Health and Human Services.

Beebe had an unusual Obamacare proposal. The Arkansas legislature did not want to expand Medicaid for those under 133 percent of the federal poverty line, an option under the Affordable Care Act. Instead, it wanted to use billions in Medicaid funding to buy private insurance for that same population.

To the surprise of many (including Beebe), the Obama administration approved, in concept, the idea.

That was just three weeks ago. Now, by my count, there are four other states considering the Arkansas approach: Florida, Ohio, Louisiana and Maine. They're the states below in yellow, alongside states with governors who have already committed to the Medicaid expansion, in pink.

This includes two governed by stalwart Obamacare opponents. Maine Gov. Paul LePage notably said, a few months ago, that he was "not lifting a finger" to implement the law in his state.

Louisiana Gov. Bobby Jindal, meanwhile, told Greta Van Susteren in July that expanding Medicaid "is a huge mistake for the country, certainly for the state of Louisiana."

There's even some chatter about pursuing such an option in Texas.

It's hard to underscore how much Arkansas's simple suggestion — that it be able to use Medicaid dollars to buy private insurance for the expansion population — has the potential to change the face of the Affordable Care Act. If these four states signed onto the Medicaid expansion through the Arkansas option, the Urban Institute estimates that 2.4 million people would gain health insurance coverage.

That works out to about 15 percent of the overall Medicaid expansion population that Urban expects to gain coverage under this part of the health care law.

We still don't know the final details of the Arkansas agreement — or whether there is a final agreement. There are questions about whether this option will be more expensive than a traditional Medicaid expansion (the public program tends to cost a few thousand dollars less per enrollee).

What we do know, though, is that there are some very strong opponents of the Obamacare law — ones that have pledged never to expand Medicaid — who suddenly find this option palatable. It has the potential to grow the Medicaid expansion by millions of Americans, edging closer to the pre-Supreme Court version of the health-law's coverage expansion.