Ohio Gov. John Kasich has announced that his state will participate in the Medicaid expansion, a move that is expected to extend coverage to 684,000 Ohio residents.

Ohio is the fifth state with a Republican governor to sign up for the health law expansion, and by far the largest: Data from the Kaiser Family Foundation suggest the Ohio expansion will cover more people than the four other states' expansions — Arizona, New Mexico, Nevada and North Dakota — combined.

What was the reasoning? The governor's budget gives a few clues. First, it's a great deal for the state: They get 100 percent funding for all those newly eligible on the expansion. It "frees up local funds for better mental health and addiction services, but it also helps prevent increases to health-care premiums and potentially devastating impacts to local hospitals," Kasich writes in his budget highlights.

The other big factor: If Ohio doesn't use the money, it looses it to other states. Participating in the expansion "avoids leaving Ohians federal tax dollars on the table and keeps the federal government from simply giving them away to other states," he writes.

Those arguments aren't exactly new ones: They've been around for about as long as the Medicaid program has existed. These two factors were the same ones Arizona used, way back in 1982, when it became the very last state to join the Medicaid program altogether.

That being said, they aren't swaying all governors. Ten states have opted out of the expansion. Eleven more, according to the Advisory Board Company, are sitting on the fence — and that includes a few with Democratic governors, like West Virginia and North Carolina.

Their decisions matter: Right now, 5.2 million Americans who would likely gain health insurance through the Medicaid expansion live in states who have signed on. Exactly double that — 10.4 million Americans — live in states that have either decided against the expansion, or are still unsure (click here for an interactive version).

There's no deadline for states to make up their minds — but a lot riding on their eventual decisions.