The Supreme Court’s decision to uphold the Affordable Care Act shifts the focus from whether sweeping changes to the health insurance market should take place to a scramble to meet the law’s rapidly approaching deadlines.


A number of largely Republican-led states that gambled on delay now face the unsettling prospect that the federal government could take over their responsibilities, particularly in setting up the health insurance marketplaces known as exchanges, where people will be able to choose among policies for their coverage.


Under the law, which the court upheld in its entirety by a 5-to-4 vote, individuals must be able to buy insurance coverage through the new state exchanges by Jan. 1, 2014. But a more immediate deadline is less than six months away, on Jan. 1, 2013, when states must demonstrate to the Department of Health and Human Services that the exchanges will be operational the next year.


If they do not, the secretary, Kathleen Sebelius, “shall establish and operate” the exchanges for the states, according to the statute, a prospect that Republican governors like Rick Scott of Florida, Rick Perry of Texas and Scott Walker of Wisconsin would presumably find anathema.


Mr. Walker quickly raised the risk by announcing that, in spite of the ruling, he would continue to delay any imposition of the law while waiting to see whether Republicans took control of the White House or Congress in November. Republicans on Capitol Hill, and the party’s presumptive presidential nominee, Mitt Romney, have vowed to repeal the entire law if they gain power.


“Wisconsin will not take any action to implement Obamacare,” Mr. Walker said in a statement. “I am hopeful that political changes in Washington, D.C., later this year ultimately end the implementation of this law at the federal level.”


Mr. Perry issued a statement calling the court’s ruling “a stomach punch to the American economy.” While he did not say whether he would move ahead to put the law in place, he echoed Mr. Walker in using the ruling as a call to political arms. “Now that the Supreme Court has abandoned us,” Mr. Perry said, “we citizens must take action at every level of government and demand real reform.”


The court also left conservative states and those with financial troubles to decide whether to participate in a significant expansion of Medicaid eligibility scheduled for 2014. The majority ruled that it was permissible for the federal government to impose such an expansion on states, and to ask that they eventually pay a fractional share of the cost of covering low-income residents.


But it ruled that the federal government could not penalize states for noncompliance by withholding their existing federal Medicaid dollars, effectively eliminating Washington’s enforcement stick.


Ohio’s Republican governor, John R. Kasich, suggested in a statement that he would examine his options. “We are very concerned that a sudden, dramatic increase in Medicaid spending could threaten Ohio’s ability to pursue needed reforms in other areas, such as education,” he said. “Going forward, we remain committed to minimizing the law’s drag on the economic growth Ohio is beginning to experience.”


But other Republican governors also seemed unsure. Mr. Scott, one of the law’s most ardent opponents, said only that he was disappointed and needed to review the ruling closely before deciding whether Florida would establish its own exchange or extend Medicaid eligibility.


With the insurance mandate upheld, the law’s major changes are now scheduled to take place by 2014, unless the forthcoming election and budget realities dictate otherwise.


Individuals who do not qualify for exemptions based on income or religion will be required to have minimum insurance coverage, or pay income tax penalties. Insurers will have to offer policies to anyone who applies, even those with expensive medical conditions, and to charge them rates comparable to healthy individuals of the same age. Under the law, those rules already apply to children.


The Medicaid expansion, along with tax credits to subsidize premiums for low-income consumers, leaves hospitals and physicians to prepare for an influx of newly insured patients.