An appeals court will hear arguments Tuesday on whether Congress effectively invalidated former President Barack Obama’s entire signature health care law when it zeroed out the tax imposed on those who chose not to buy insurance.

It’s unclear when the three-judge panel of the 5th U.S. Circuit Court of Appeals panel will rule in a case that appears destined for the Supreme Court, which has reviewed the law, and its coverage and insurance protections for millions of Americans, before. The ultimate outcome will affect protections for people with pre-existing conditions, Medicaid expansions covering roughly 12 million people, and subsidies that help about 10 million others afford health insurance.

Tuesday’s arguments are the latest in a lawsuit filed by Republican officials in 18 states, led by the Texas Attorney General’s Office. It was filed after Congress ? which didn’t repeal the law, despite pressure from President Donald Trump ? reduced to zero the unpopular tax imposed on those without insurance.

In challenging the law anew, “Obamacare” opponents noted the 2012 ruling of a divided Supreme Court that upheld the law. Conservative justices had rejected the argument that Congress could require everyone to buy insurance under the Constitution’s interstate commerce clause. But Chief Justice John Roberts, joining four liberal justices, said Congress did have the power to impose a tax on those without insurance.

With no tax penalty now in effect, the Texas lawsuit argues, the individual mandate is unconstitutional and the entire law must fall without it. Texas-based U.S. District Judge Reed O’Connor agreed in a December ruling. The law’s supporters appealed.

In addition to the 18 states, two individual taxpayers are part of the lawsuit. The Trump administration is not defending the law and has filed arguments in favor of O’Connor’s ruling.

California’s attorney general represents a coalition of mostly Democratic-led states and the District of Columbia seeking to overturn O’Connor’s ruling and uphold the law. The House of Representatives has joined them. Among the arguments by the law’s supporters: Those who filed suit have no case because they aren’t harmed by a penalty that doesn’t exist; the reduction of the tax penalty to zero could be read as a suspension of the tax, but the tax’s legal structure still exists; and that, even if the individual mandate is now unconstitutional, that does not affect the rest of the law known as the Affordable Care Act.