As Americans shop in the health insurance marketplace for a second year, President Obama is depending more than ever on the insurance companies that five years ago he accused of padding profits and canceling coverage for the sick.

Those same insurers have long viewed government as an unreliable business partner that imposed taxes, fees and countless regulations and had the power to cut payment rates and cap profit margins.

But since the Affordable Care Act was enacted in 2010, the relationship between the Obama administration and insurers has evolved into a powerful, mutually beneficial partnership that has been a boon to the nation’s largest private health plans and led to a profitable surge in their Medicaid enrollment.

The insurers in turn have provided crucial support to Mr. Obama in court battles over the health care law, including a case now before the Supreme Court challenging the federal subsidies paid to insurance companies on behalf of low- and moderate-income consumers. Last fall, a unit of one of the nation’s largest insurers, UnitedHealth Group, helped the administration repair the HealthCare.gov website after it crashed in the opening days of enrollment.

“Insurers and the government have developed a symbiotic relationship, nurtured by tens of billions of dollars that flow from the federal Treasury to insurers each year,” said Michael F. Cannon, director of health policy studies at the libertarian Cato Institute.

So much so, in fact, that insurers may soon be on a collision course with the Republican majority in the new Congress. Insurers, often aligned with Republicans in the past, have built their business plans around the law and will strenuously resist Republican efforts to dismantle it. Since Mr. Obama signed the law, share prices for four of the major insurance companies — Aetna, Cigna, Humana and UnitedHealth — have more than doubled, while the Standard & Poor’s 500-stock index has increased about 70 percent.

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Jeffrey R. Ungvary President