WILMINGTON, Del. (AP) — President-elect Joe Biden fought to confront the nation's competing crises — and fierce Republican resistance — on Tuesday as concerns grew that President Donald Trump's refusal to acknowledge his Democratic rival's victory could undermine national security.

Raising unsupported claims of voter fraud, Trump has blocked Biden from receiving the intelligence briefings traditionally shared with incoming presidents, according to someone with knowledge of the situation but not authorized to disclose private conversations. Trump's resistance, backed by senior Republicans in Washington and across the country, could also prevent background investigations and security clearances for Biden’s prospective national security team and access to federal agencies to discuss budget and policy issues.

Despite growing frustration, Biden was scheduled to deliver an afternoon speech on the Affordable Care Act, just hours after the Supreme Court heard arguments on its merits. The high court ruled eight years ago to leave intact the essential components of the law known as “Obamacare,” but Trump and his Republican allies are seeking to have it overturned.

If the 6-3 conservative court ultimately agrees with the GOP, millions of Americans could lose their health care coverage. While Tuesday's arguments indicate the court is unlikely to strike down the entire law, the prospect adds to the pressure on Biden to execute complicated plans to confront a series of crisis he will inherit in just 71 days. The nation's economy is struggling as the pandemic surges and cultural divisions deepen.

Trump and his allies seemed determined to make Biden's transition as difficult as possible.

From his Twitter account on Tuesday, Trump again raised unsupported claims of “massive ballot counting abuse” and predicted he would ultimately win the race he has already lost. His allies on Capitol Hill, led by Senate Majority Leader Mitch McConnell, have encouraged the president's baseless accusations.

America's allies began to acknowledge what Trump would not.

French President Emmanuel Macron met with Biden via video conference and German Chancellor Angela Merkel and British Prime Minister Boris Johnson also congratulated Biden on his election.

“I look forward to strengthening the partnership between our countries and to working with him on our shared priorities – from tackling climate change, to promoting democracy and building back better from the pandemic,” Johnson wrote on Twitter. “Build back better” is a slogan that Biden and the British government have in common.

Meanwhile, Biden tried to stay focused on health care as he prepares to take office on Jan. 20 during the worst health crisis in more than a century. The U.S. surpassed 10 million cases on Monday and cases are skyrocketing as the nation moves into the cold winter months.

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.

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

Complicating Biden's challenge is the Republican Party's widespread refusal to acknowledge his victory. With scant evidence, Trump and his allies are insisting that the election was stolen.

Attorney General William Barr has authorized the Justice Department to probe unsubstantiated allegations of voter fraud. And 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. The GSA inaction could continue to deny Biden security briefings, which he received periodically before the election, as well as delaying security clearances and staffing decisions.

Senior officials in the George W. Bush administration warned that a similar delay after the closely contested 2000 presidential election caused many difficulties.

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.”

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Peoples reported from New York. AP writers Mary Clare Jalonick and Lisa Mascaro in Washington contributed.