One of the nation’s most conservative federal appeals courts heard opening arguments Tuesday in a case that could determine the future of the Affordable Care Act and seemed destined to make its way to the Supreme Court.

The three-judge panel of the 5th U.S. Circuit Court of Appeals in New Orleans heard arguments in Texas vs. the United States, which seeks to determine 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.

Speaking on behalf of Texas Attorney General Ken Paxton and the 18 other states involved in the case, the state’s Solicitor General Kyle Hawkins urged the panel to rule that ObamaCare is unconstitutional in its entirety, arguing that when Congress enacted President Trump’s tax overhaul, it rendered ObamaCare unconstitutional by doing away with the tax penalty for violating the plan’s individual mandate.

It was unclear when the court would issue a ruling or if it will actually make it to the Supreme Court, but 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.

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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.

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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 even if the individual mandate is now unconstitutional, that does not affect the rest of the law known as the Affordable Care Act.

When the law was proposed, friends and foes of ObamaCare agreed that the tax was essential to persuade healthy people to get insured, thereby keeping premiums in check. But this year -- the first time no fines will be collected -- the number of people signing up for subsidized private insurance through the Affordable Care Act slipped only slightly.

The government said in March that a total of 11.4 million people signed up during open enrollment season, a dip of about 300,000 from last year.

The judges hearing the arguments were 5th Circuit Judges Carolyn Dineen King, Jennifer Walker Elrod and Kurt Engelhardt. King was nominated to the appeals court by President Jimmy Carter in 1979. Elrod was nominated by President George W. Bush in 2007. Engelhardt was nominated by President Donald Trump last year.

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Democratic leaders in both the Senate and the House on Tuesday warned their Republican colleagues of the costs – both political and financial – the loss of the popular ObamaCare would incur.

“These are faces of Americans, millions who will suffer if Republicans succeed in dismantling health care law,” Senate Minority Leader Chuck Schumer, D-N.Y., said during a press conference Tuesday alongside House Speaker Nancy Pelosi, D-Calf. “If the right win lawsuits, families will lose their healthcare.”

Fox News' Ronn Blitzer and Ellen Smith and The Associated Press contributed to this report.