WILMINGTON, Del. — 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.

Joe Biden defeated Trump and was declared the 46th President of the United States on Saturday morning.

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 crises he will inherit in 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.

Election crimes chief resigns after AG Barr authorizes voter fraud probes

Election crimes chief resigns after AG Barr authorizes voter fraud probes

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 Jan. 20 during the worst health crisis in more than a century. The U.S. surpassed 10 million cases Monday, and cases are skyrocketing as the nation moves into winter.

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 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.