The Supreme Court agreed Friday to hear a new challenge to ObamaCare, bringing the law back before the court after it survived a brush with death in 2012. 

At issue in this case is the legality of subsidies offered to help millions of low- and middle-income people buy health insurance. Opponents argue that most of the subsidies are illegal.

A federal appeals court upheld Internal Revenue Service regulations that allow health-insurance tax credits under the Affordable Care Act for consumers in all 50 states. 

But opponents of the subsidies argued the Supreme Court should resolve the issue now because it involves billions of dollars in public money. At least four justices, needed to grant review, apparently agreed with the challengers that the issue is important enough to decide now.

The case is likely to be heard in the spring of 2015. 

"The plain language of the law makes it clear that subsidies are only to be provided for the purchase of health coverage through exchanges setup by the states," Rep. Tom Price, R-Ga., said in a statement. "Nevertheless, the Obama administration and others are asking the courts to disregard the letter of the law and instead rule based on bureaucratic rewrites and revisions." 

White House Press Secretary Josh Earnest said the congressional intent behind the law is for eligible customers regardless of where they live to receive assistance from the government to subsidize the purchase of health care. He promised a vigorous defense before the high court. 

"The ACA is working. These lawsuits won't stand in the way of the Affordable Care Act and the millions of Americans who can now afford health insurance because of it," he said in a statement, calling the lawsuit "just another partisan attempt to undermine the Affordable Care Act and to strip millions of American families of tax credits that Congress intended for them to have." 

The justices upheld the heart of the law in a 5-4 decision in 2012 in which Chief Justice John Roberts provided the decisive vote, preserving the law's individual mandate to buy insurance. 

This past June, the court again ruled on ObamaCare, this time siding with companies that had religious objections over the law's requirement to provide contraceptive coverage. The ruling forced the administration to adjust the regulations, but did not seriously disrupt the health law. 

The case over insurance subsidies, though, puts more at stake for the administration. The insurance subsidies are a key plank of the law's system for ensuring that the people required to buy insurance can actually afford to pay for it. Foes have challenged the legality of providing them in states that do not have their own insurance exchanges -- in other words, those using HealthCare.gov. 

In July, a Richmond, Virginia-based appeals court upheld Internal Revenue Service regulations that allow health-insurance tax credits under the Affordable Care Act for consumers in all 50 states.

On that same July day, a panel of appellate judges in Washington, District of Columbia, sided with the challengers in striking down the IRS regulations. The Washington court held that under the law, financial aid can be provided only in states that have set up their own insurance markets, known as exchanges.

The administration said in court papers that the federal government is running the exchanges in 34 states and that nearly 5 million people receive subsidies that allow them to purchase health insurance through those exchanges.

For those federal exchange consumers, the subsidies cover 76 percent of their premiums, on average. Customers now pay an average of $82 on total monthly premiums averaging $346. The federal subsidy of $264 a month makes up the difference.

But in October, the entire Washington appeals court voted to rehear the case and threw out the panel's ruling, eliminating the so-called circuit split. The appeals argument has been scheduled for December 17, but that case now recedes in importance with the Supreme Court's action to step in.

The court rarely steps into a case when there is no disagreement among federal appellate courts, unless a law or regulation has been ruled invalid.

Fox News' Shannon Bream and The Associated Press contributed to this report.