WILMINGTON, Del. (AP) — Facing fierce Republican refusal to acknowledge his victory, President-elect Joe Biden was fighting to defend the Obama administration's signature health care law on Tuesday as he began to confront the crises he will inherit in just 71 days.

Biden was scheduled to deliver an afternoon speech on the Affordable Care Act on the same day the Supreme Court hears arguments on its merits.

The high court ruled eight years ago to leave intact the essential components of the law known as “Obamacare,” but President Donald Trump and his Republican allies are arguing to have it overturned. If the 6-3 conservative court agrees with the GOP, millions of Americans could lose their health care coverage.

Biden's speech reflects his steady focus on health care as he prepares to take office on Jan. 20 during the worst health crisis in more than a century. One of Biden's chief coronavirus advisers, former Surgeon General Vivek Murthy, planned to brief Senate Democrats on Tuesday by phone at their weekly virtual lunch, according to a senior Democratic aide granted anonymity to discuss the private session.

The closed-door meeting marks the first time a Biden transition official has addressed the full Senate caucus since last week's election.

Complicating Biden's challenge is the Republican Party's widespread refusal to acknowledge his victory. Trump and his allies have raised baseless charges of voter fraud, insisting that the election was stolen. The Republican president has been blocking government officials from cooperating with the president-elect's team, while Attorney General William Barr authorized the Justice Department to probe unsubstantiated allegations of voter fraud.

Trump's wild claims have little merit, but they could create real problems for the incoming transition at a dangerous moment. Beyond the pandemic, Biden will inherit a struggling U.S. economy and a nation deeply divided along racial and cultural lines.

So far, the General Services Administration, led by a Trump-appointed administrator, Emily Murphy, has declined to formally recognize Biden as president-elect. That designation eases cooperation between the outgoing and incoming administrations, although Murphy has not started the process and has given no guidance on when she will.

On a call Monday night with reporters, a transition official said Biden's team believes it is time for the GSA administrator to ascertain that he is president-elect. The official, who spoke only on condition of anonymity as a ground rule for the call, said legal action is “certainly a possibility” if that doesn’t happen, though there are also other options being considered.

That lack of clarity is fueling questions about whether Trump will spend as much of his remaining time in office as possible hindering Biden from building out his government. Biden has largely ignored Trump and instead called for bipartisanship — but he may not be able to keep doing so for much longer.

“This election is over,” Biden said during his speech Monday. “It’s time to put aside the partisanship and the rhetoric that is designed to demonize one another.”

Meanwhile, Biden is expected to quickly name a chief of staff and start considering Cabinet appointments, though those likely won’t be finalized for weeks. He was spending Tuesday working alongside Vice President-elect Kamala Harris at a theater near his home in downtown Wilmington.

The suit challenging the health care law was brought in America's largest conservative state, Texas, and is backed by Trump and top Republicans. It asks the Supreme Court to declare the law’s mandate to buy health insurance unconstitutional because Congress had previously repealed the penalties for noncompliance.

After serving as President Barack Obama's vice president for eight years, Biden has pledged to build on the Affordable Care Act while championing a “public option” that would allow more people to opt into government-sponsored health insurance even as millions of others could stick with their current, usually employer-based coverage.

But such changes could be difficult to enact if Democrats fail to win a majority in the Senate. Control of the chamber hinges on two runoff races in Georgia that will be decided in January.

Biden also focused on health care Monday as he pleaded with Americans to put aside their political differences and wear masks to protect themselves and their neighbors from the virus.

The pandemic has dominated Biden’s attention in the early days of his transition planning as the coronavirus surges across the country. The U.S. surpassed 10 million cases on Monday and cases are skyrocketing as the nation moves into the cold winter months.

“We could save tens of thousands of lives if everyone would just wear a mask for the next few months. Not Democratic or Republican lives, American lives,” Biden said. “Please, I implore you, wear a mask.”