President Barack Obama’s Supreme Court victory last week came with a significant asterisk.

A pillar of Obama’s health-care law — the dramatic expansion of Medicaid to cover millions of low-income adults — is now optional for the nation’s 50 states. The court upheld the expansion of Medicaid to a projected 17 million more Americans but struck down a provision that would have forced states to do it.

There is growing chatter that this risks further splitting the country into haves and have-nots, depending on the politics of your state. Red states, the theory goes, may forgo billions in new federal dollars to expand Medicaid, citing an unwelcome federal intrusion. Meanwhile, blue states will move forward with the expansion.

There is also a risk that even blue states, such as Illinois, will reject a Medicaid expansion because their cash-strapped states can’t afford it. It was encouraging to hear Gov. Pat Quinn on Thursday restate his commitment to Medicaid expansion, but no one man governs indefinitely or alone.

Neither of these two scenarios should play out without a fight. Expanding health-care access is central to the promise of Obama’s Affordable Care Act. The Congressional Budget Office estimates that the Medicaid expansion would account for about half the total projected coverage increases under the law. Without expanding Medicaid to include more poor adults, new coverage will be guaranteed only to middle-income Americans through new state-run insurance exchanges.

That is not the vision that carried ObamaCare, nor is it a vision of a more just America.

But the problems of cost and coercion are real and must be worked out before the Medicaid expansion begins in 2014.

The strongest argument in favor of expanding Medicaid comes down to who pays.

From 2014 to 2017, Washington will pay 100 percent of the cost. That will drop gradually, leveling off at 90 percent in 2020, leaving the states to pay just 10 percent of a new entitlement that undoubtedly will bring other economic benefits to each state.

Illinois will get about $19.3 billion in federal funds for newly eligible Medicaid recipients between 2014 and 2019, according to an analysis by the Urban Institute for the Kaiser Commission on Medicaid and the Uninsured. In return, Illinois will have to chip in $1.2 billion.

That is not insignificant for a state like Illinois, which just last month was forced to cut $1.6 billion of its existing Medicaid program. It’s also crucial that the federal government stay true to its 90 percent funding commitment.

With sufficient planning and political will, states can pay that 10 percent — no matter their political persuasions. The rewards, in terms of healthier citizens, healthier local economies and a healthier America, will far outweigh the costs.