Kentucky Gov. Matt Bevin stormed into office promising to kill off the state's Obamacare Medicaid expansion immediately.

"Absolutely. No question about it. I would reverse that immediately," Bevin said in February.

But since getting sworn in this past December, his tune has changed — a lot. The Republican governor won't undo his Democratic predecessor's work expanding Medicaid to cover half a million of the state's low-income residents.

"It would be damaging to low-income adults in Kentucky to go from expansion to no expansion"

Instead, he quietly announced (right between Christmas and New Year's) that he would instead look at ways to "transform the way Medicaid is delivered." Changing delivery of the program is a whole lot different from eliminating it.

Kentucky was the first state in the country where a Republican governor won on a platform of undoing his Democratic predecessor's Medicaid expansion. The fact that Bevin has decided to drop the issue suggests something important about the politics of Obamacare: Once a state uses the health law to expand its Medicaid coverage, it's incredibly difficult to shrink the benefit back down.

Kentucky was one of the first states to expand Medicaid

Democratic governors were quick to embrace the Medicaid expansion in 2014, after a Supreme Court decision made the coverage optional.

Republican governors moved slower but over the past two years have begun to warm up to the idea of using federal money to buy public health coverage for their lowest-income residents.

Under the leadership of Democratic Gov. Steve Beshear, Kentucky was among the first wave of states to expand Medicaid.

Kentucky ended up having one of the most successful Obamacare launches. The state won great praise for its marketplace, Kynect, which actually worked when open enrollment started (unlike the federal marketplace, Healthcare.gov).

The state also expanded Medicaid from day one. Those two actions, taken together, have helped Kentucky quickly drive down the rate of uninsured people. The state's uninsured rate has declined 55 percent over the past two years — the sixth-fastest drop in the country.

The Medicaid expansion is going well for enrollees — way better than getting nothing at all

Since 2013, Kentucky's Medicaid program has nearly doubled its enrollment.

The best available data suggests that the program is helping low-income Kentuckians fare much better than they would in a world that Bevin initially suggested — one in which Medicaid expansion was wiped off the books.

On Tuesday, Ben Sommers, a public health professor at Harvard University, published a study comparing the low-income residents of Kentucky with those of Texas, which has not expanded Medicaid. His research in the journal Health Affairs estimates that Kentucky's Medicaid expansion was responsible for a 16.6 percent decline in the uninsured rate for low-income residents.

And that insurance seemed to help Kentuckians. Sommers and his co-authors surveyed hundreds of Kentucky and Texas's low-income residents before and after the expansion. The percentage of Kentuckians who said they had skipped medicines due to cost or had trouble paying their health care bills dropped. Their access to care improved, too, particularly for the management of chronic diseases like diabetes or hypertension.

"It's pretty clear from our study that it would be damaging to low-income adults in Kentucky to go from expansion to no expansion," Sommers says. "It would introduce huge affordability issues."

Reversing course on Medicaid expansion is hard

It's one thing to decline the Medicaid expansion. There are 20 Republican governors in office who have made this decision and aren't facing any serious political blowback.

But the Bevin situation shows that reversing course on Medicaid expansion has a completely different dynamic: There are half a million Kentuckians who rely on the program, who have an easier time paying their health care bills, and who would face difficulty accessing doctors if the expansion disappeared. Those people don't exist in Texas, which never expanded the program. But they exist in Kentucky, and that matters.

This underscores how crucial the first decision to expand Medicaid becomes and the legacy it creates. That's powerful for Obamacare supporters, who are pushing more states to expand. They most likely won't have to lobby, over and over again, for states to continue their Medicaid expansion.

This was, in part, Democrats' political theory of Obamacare: The more the law becomes a system that Americans rely on, the harder it will become for future politicians to dismantle it. In Kentucky, that theory is playing out in real time — and it's working to Obamacare supporters' advantage.