WASHINGTON – Pop quiz: The Affordable Care Act became law in 2010, was upheld by the Supreme Court in 2012 and 2015, and has survived dozens of repeal efforts in Congress. So is it finally safe?

Answer: Not yet.

The latest threat looms in New Orleans, where a three-judge panel of the U.S. Court of Appeals for the Fifth Circuit will hear 90 minutes of oral argument Tuesday in a challenge that threatens the law's key features – and possibly the entire 974-page statute.

We have been here before: in 2012, when the Supreme Court ruled 5-4 that the law, known as Obamacare, was constitutional under Congress' power of taxation; and again in 2015, when it saved the law's critical tax credits in federal as well as state insurance exchanges.

But in December, federal District Judge Reed O'Connor ruled that by repealing the tax on people who refuse to buy insurance, Congress in 2017 rendered the individual mandate unconstitutional and, by extension, the law itself. The repeal was included in the $1.5 trillion tax cut pushed through the Republican-controlled Congress and signed by President Donald Trump.

That ruling has not been implemented pending appeal, but the implications are huge. If the law is wiped out, so too would be insurance for 20 million people, protection for people with preexisting conditions, subsidies for low-income people, Medicaid expansions in many states, coverage for young adults up to age 26 and more.

Now comes Step 2 of the latest court battle, one that could bring the Affordable Care Act yet again to the Supreme Court.

On one side are the Trump administration and 18 states, led by Texas, that agree with O'Connor's decision and want the law dismantled. On the other side are the U.S. House of Representatives and 16 states, led by California, seeking to have that ruling reversed. A threshold question is whether the House, and possibly the liberal states, have the required legal authority to defend a law the federal government is not defending.

Hearing the two sides will be a three-judge panel that includes two judges chosen by Republican presidents and one by a Democrat. If its decision eventually goes to the full appeals court, that too is dominated by GOP presidents' choices.

The betting line among most legal experts is that a decision reversing the trial judge and allowing the health care law to survive would not be heard by the Supreme Court, which already has spoken on the law's constitutionality. But if the appeals court agrees that the law should be struck down, the high court is virtually certain to hear an appeal – possibly next year.

'The last leg it stood on'

The battle lines were three-sided before the December district court ruling. While the two sets of states argued for the Affordable Care Act's survival or demise, the Trump administration wanted only the individual mandate and associated insurance changes quashed, including protections and cost controls for people with preexisting conditions.

In his decision, O'Connor said the intentions of both the 2010 and 2017 Congresses had to be considered. "The former enacted the ACA," he said. "The latter sawed off the last leg it stood on."

Since then, the administration has joined the conservative states in calling for the entire law to be dismantled. Assembling something to take its place would be left to Congress and individual states.

"In the district court, the Department of Justice took the position that the remainder of the ACA was severable," government lawyers told the appeals court. "But upon further consideration and review of the district court’s opinion, it is the position of the United States that the balance of the ACA also is inseverable and must be struck down."

That's the position of Texas and other conservative states. "In the end, the ACA is a naked command to buy an insurance product the government deems suitable," Texas Solicitor General Kyle Hawkins argued in court papers.

"Bereft of penalties, the mandate now raises no revenue and therefore cannot by any conceivable definition be considered a tax," he added. "Stripped of its tax status, the individual mandate is nothing more than an unconstitutional congressional mandate to purchase health insurance."

'Thrown into chaos'

Lawyers for California and other liberal-leaning states defend the law first by stating its policy achievements. "The health of millions of Americans has improved," they said in court papers.

By eliminating the tax penalty for those who do not buy insurance, Congress in 2017 merely subtracted a requirement without harming anyone, the states argued.

"A provision that offers individuals a choice between buying health insurance and suffering no legal consequences for not doing so neither imposes any legal injury nor violates the Constitution," California Attorney General Xavier Becerra said.

Donald Verrilli, the former U.S. solicitor general who successfully defended the health care law at the Supreme Court in 2012 and 2015, is representing the House of Representatives in the latest case. He said O'Connor's ruling, if allowed to stand, would be "devastating."

"Millions of Americans will be denied affordable health care. Insurance costs will skyrocket. Medicare recipients will face steep increases in the price of drugs and other services," Verrilli warned in court papers. "The nation’s healthcare system will be thrown into chaos."