Donald Trump accepted the Republican presidential nomination with a marathon speech that began with an uncharacteristic call for national unity before veering into his familiar mix of grievance, bombast and apocalyptic warnings about the country's fate if he is not returned to the White House.

Trump's coronation before an adoring audience on Thursday stood in contrast to the turmoil roiling the campaign of President Joe Biden, his opponent in the Nov. 5 election, who was "soul searching" about whether to abandon his reelection bid under pressure from fellow Democrats, a source said.

Speaking to his party's national convention in Milwaukee, Trump delivered a dramatic account of the attempt on his life at a Pennsylvania rally five days ago, describing how he put a hand to his ear after hearing a bullet whiz by and saw blood.

When he told the crowd that he was "not supposed to be here," the delegates chanted back, "Yes you are!" With photos of a bloodied Trump projected behind him, he praised the Secret Service agents who rushed to his side.

Trump struck an unusually conciliatory tone during the speech's opening moments.

"I am running to be president for all of America, not half of America," he said, a marked shift in tenor for the typically bellicose former president.

But he quickly abandoned the message of unity he had promised in the wake of the shooting, pivoting to well-worn attacks on the Biden administration.

He claimed without evidence that his criminal indictments were part of a Democratic conspiracy, predicted that Biden would usher in "World War Three" and described what he called an "invasion" of migrants over the southern border.

The meandering address capped a four-day event during which he was greeted with adulation by a party now almost entirely in his thrall.

Trump devoted much of his record-breaking 92-minute speech to attacking migrants, a theme that has always animated his campaigns.

"They're coming from prisons, they're coming from jails, they're coming from mental institutions," he said, before citing by name several Americans murdered by suspects in the country illegally.

There is no evidence foreign governments are intentionally sending such people to the U.S. Academic studies show that immigrants do not commit crime at a higher rate than native-born Americans.

The speech broke Trump's own 2016 record for the longest delivered by a nominee, according to the American Presidency Project at the University of California in Santa Barbara. He also had the third longest, in 2020.

Biden campaign chair Jen O'Malley Dillon said in a statement that Trump "sought to find problems with America, not solutions."

With Biden forced off the campaign trail due to a COVID-19 diagnosis, Trump seized the spotlight on Thursday as only the former reality television star could, emerging from behind a screen in front of a set of massive spotlights spelling out his name.

The entrance evoked that of a professional wrestler, just moments after the wrestler Hulk Hogan tore his shirt in half to reveal a sleeveless red Trump campaign shirt underneath.

Hogan wasn't the only celebrity to occupy a primetime slot. After the musician Kid Rock performed "American Bad Ass," Dana White, the chief executive of Ultimate Fighting Championship, introduced Trump as "the toughest, most resilient guy that I've ever met in my life."

After Trump concluded, his family and that of his 39-year-old running mate, Senator J.D. Vance, shared the stage as balloons dropped from the ceiling.

With his grip on the Republican Party never tighter, Trump is in a much stronger position than in his 2017-2021 term to follow through on his agenda if he wins the election.

Isolating at his Rehoboth Beach home in Delaware because of COVID, Biden faces increasing pressure from party leaders to cede his position at the top of the ticket following a halting debate performance on June 27. 

Former House of Representatives Speaker Nancy Pelosi is among those who have told him he cannot win in November, according to a White House source familiar with the matter.

After weeks of insisting he will remain in the race, Biden is now taking calls to step aside seriously, and multiple Democratic officials think an exit is a matter of time, sources said.

Senator Jon Tester, who faces a challenging reelection battle in Montana this year, on Thursday became the 21st congressional Democrat to publicly call on Biden to drop out.