WASHINGTON — Starting in 2014, millions of uninsured Americans should be able to get health coverage. Millions more who don’t get insurance through work should be able to buy a new basic plan.

“It’s all over but the shouting,” said Families USA Executive Director Ron Pollack, an influential consumer advocate and leading champion of the law. “What was very questionable at the start of the year has been settled. … The Affordable Care Act will be a permanent fixture of the American health care system.”

That outcome puts immediate pressure on many Republican state leaders who fought it, including in Idaho. They must decide in days how to implement it or have the federal government do it for them.

Tuesday’s results also present President Barack Obama with a new set of challenges as he tries to fulfill the promise of the biggest expansion of the social safety net since Medicare and Medicaid were created in 1965.

Federal and state officials must create systems to handle millions of new insurance customers. Key to that will be setting up insurance exchanges in every state by next October.

Obama also will face renewed pressure to scale back the law as Congress tries to rein in federal budget deficits.

The act authorizes more than $1 trillion in new federal spending over the next decade. Although that is offset with new taxes and other spending cuts, critics say the law’s program to provide insurance subsidies to households making up to four times the federal poverty level — or about $90,000 for a family of four — is overly generous in a time of huge budget woes.

At the same time, health insurance companies, hospitals, employers and other interest groups are gearing up lobbying campaigns to modify the law, parts of which threaten to increase health care costs.

Also unclear is how the administration will contend with GOP governors who continue to resist the law and might spurn hundreds of billions of federal dollars to provide insurance coverage.

Several states, including Texas, Florida and Louisiana, have indicated that they will not open their Medicaid programs in 2014 to cover all low-income citizens. Most states — including those with Republican statehouses — are expected to expand Medicaid because the law provides hundreds of billions of dollars in federal money.

The president could face even more resistance to the law if insurance premiums and other medical costs continue to rise in coming years, undermining a promise he made in pushing for reform.

“It will be full-speed ahead with implementation. But there could still be some rocky going,” predicted Dean Rosen, a Republican health care lobbyist and onetime aide to former Senate Majority Leader Bill Frist.

James Capretta, associate director of the White House Office of Management and Budget under President George W. Bush, said many conservatives will not stop contesting the law. “The fight over the size and scope of government will continue,” he said. “Giving up is not an option.”

Polls show the public is divided over the legislation. Most Republicans remain strongly in favor of repealing it. And with control of the House, GOP lawmakers could continue to push proposals to eliminate or roll back parts of the law.

Obama in the past has signaled willingness to modify the law, but he has flatly rejected any major retrenchments.

Administration officials also insist they will not delay the law, which will allow Americans in all 50 states who don’t get coverage through work to buy insurance on the Internet-based exchanges.

Under the law, states have until Nov. 16 to tell the department whether they will set up an exchange, a complicated project requiring new data systems, regulations and bureaucracy.

Just 15 states, including California, Maryland and Connecticut, have established an exchange, according to the nonpartisan Kaiser Family Foundation. More than a third of the states are expected to reject that option, forcing the federal government to step in.

There has been much hand-wringing among state officials and others over the slow process of writing regulations outlining how these federal exchanges will work. But several experts said it’s highly likely the Obama administration will be ready to open exchanges next fall.

Heather Howard, who directs the State Health Reform Assistance Network at Princeton University, cited Washington’s recent experience creating the popular Medicare Part D prescription drug program.

More uncertain is what smaller modifications the Obama administration might agree to make.

Insurance companies have been warning that a new tax on insurers — scheduled for 2014 — will be passed on to consumers, further inflating health care costs. Many insurers also warn that regulations limiting how much more insurers can charge older consumers could mean much higher rates for young people.

Many hospitals, meanwhile, are worried that they could be saddled with more uninsured patients as states with conservative leaders decline to extend Medicaid coverage.

And employers are stepping up warnings that the law’s mandate requiring them to provide health benefits to full-time employees might prompt some to shift more workers to part-time, thereby depressing employment.