A ruling against the law also would threaten other planned expansions, such as in Washington state, which received $128 million to help set up its insurance exchange and an additional $200 million for its preexisting-condition insurance pool. Officials are uncertain whether that money could stay with the state or would have to be returned.

In Michigan, the law has brought widespread benefits: More than half a million seniors have received some free preventive health care services this year. And 1.8 million residents now receive preventative services with no co-pay. Another 57,000 young adults in Michigan are on their parents’ health insurance plans.

Last week, U.S. Health and Human Services director Kathleen Sebelius visited Detroit to announce grants for six Michigan health centers to help expand access to care under the law. Officials there aren’t sure what they'll do if that money disappears.

Funding for North Dakota’s 15 community health centers may be jeopardized, and three other planned centers might not be built if the federal law is overturned.

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SWEATING DEADLINES:

Most states have taken few — if any — steps toward creating the insurance exchanges that are a cornerstone of the health law. The online marketplaces would allow people and small businesses to comparison-shop for health insurance.

So if the court upholds the mandate to establish exchanges, many states will have to scramble to get their plans approved by January and the programs running a year later. As of March, only 13 states and Washington, D.C., had adopted plans for exchanges.

In Obama’s home state of Illinois, Democratic Gov. Pat Quinn considered an executive order to establish an exchange, but state Rep. Frank Mautino said it’s too late. The first year of the project will have to be a joint state-federal exchange, he said.

In Michigan, House Republicans have refused to let state officials start setting up the MI Health Marketplace before the Supreme Court ruling. Gov. Rick Snyder is one the few Republican leaders who has acknowledged considering a state-federal partnership to get an exchange in place.

‘‘I'm just trying to be a pragmatist here,’’ Snyder said Tuesday.

In Indiana, Republican Gov. Mitch Daniels ordered state agencies to investigate the cost and feasibility of an online health insurance marketplace in January 2011, but Daniels told the agencies to keep their work in the planning stages until the court ruled.

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