President Donald Trump's lame-duck administration makes a last try in the US Supreme Court Tuesday to demolish the "Obamacare" health program, which could cancel the health insurance of millions in the middle of a pandemic.

The high court will hear arguments in the long-brewing case over the constitutionality of the 2010 Affordable Care Act, under which then-president Barack Obama's government sought to extend health insurance to people who could not afford it. 

Since taking office in 2017, Trump has tried to undermine the ACA, first eroding one key provision through legislation, and then seeking to cancel it altogether, backing a lawsuit by Texas and several other Republican-led states.

After federal district and appeals courts sided with Texas, the case was appealed earlier this year to the nine justices of the Supreme Court by California and states which support the ACA.

- 20 million could lose insurance -

Trump's adding three conservative justices to the court since 2017 could be enough to strike down ACA legislation.

But doing so could be pyrrhic.

More than 20 million Americans are insured under the law, including poor adults who became able to access the Medicare program normally open only to retired people over 65; young people 26 years old or less the ACA allowed to be covered by their parents' insurance; and people whose preexisting medical conditions led to their being denied commercial health insurance.

And the legal challenge comes during the Covid-19 pandemic which has seen more than 10 million Americans infected with the virus.

As much as he has attacked the ACA, trying to cut related budgets and services, Trump has not yet offered alternatives that would cover those losing insurance if the law is struck down. 

"Obamacare will be replaced with a MUCH better, and FAR cheaper, alternative if it is terminated in the Supreme Court," Trump claimed by tweet in September.

- Focus on Barrett -

Tuesday's hearing comes a week after Trump was defeated at the polls for a second term by Democrat Joe Biden.

Biden was Obama's vice president when the ACA was established, and has promised to improve it.

While the program has proven popular, it stands at risk on technical legal grounds, at a Supreme Court which has turned sharply to the right since Trump came to power. 

In 2012 the court voted, 5-4, to uphold the law as constitutional. Chief Justice John Roberts, a moderate conservative, sided with the court's four liberals to decide the case.

But since then Trump has named three new justices, all conservatives. That, even without Roberts' vote, gives them five votes out of nine.

All eyes will be on the newest justice, Amy Coney Barrett, whose appointment Trump rushed through in October, saying he wanted her in position to help decide any election cases as well as the ACA case.

When still a law professor, she criticized Roberts' reasoning in the 2012 decision to uphold the ACA.

She was pressed intensely by Democrats during her Senate confirmation hearings last month on her view of the program and Trump's expectations of her.

"I'm not hostile to the ACA," she said. "I made no promises to anyone. I don't have any agenda."

Whatever the court decides, it won't likely be known until next year, possibly after Trump leaves office and Biden is president on January 20.