Donald Trump will need to deliver the speech of his life Thursday, seeking to salvage a fractious Republican convention after his chief rival declined to endorse him for president.
The most controversial White House contender in modern times will accept the nomination of the party of Abraham Lincoln, which has guided more candidates to the Oval Office than any other.
Nationwide polls put the New York mogul, who has never held elected office, almost neck and neck Hillary Clinton, the former secretary of state heavily criticized over an email scandal.
The four-day Republican convention in Cleveland, which braced for violent protests, has passed off with only a handful of arrests.
But inside the halls, the convention itself has been anything but uneventful, with public spasms of disunity -- and the embarrassing revelation that a prime-time speech by Trump's wife Melania had plagiarized remarks made by First Lady Michelle Obama.
When Trump takes center stage on Thursday night, watched by tens of millions of Americans on prime-time television, he will need to prove that he is worthy of the White House and capable of being commander-in-chief.
Many Republicans will be watching to see if he tries to heal deep party divisions, laid bare late Wednesday when his main rival Ted Cruz was booed off stage, or demands dissidents fall into line.
The speech will also sound the firing gun on the general election, offering Trump a chance to overcome voters' concerns about his divisive campaign rhetoric.
His campaign has defied political norms -- fueling ethnic tensions, offending key voting blocs, eschewing big-spending ad buys or campaign infrastructure and relying on heavy media coverage.
"Mr Trump's speech will focus on his vision," his campaign chairman Paul Manafort said Thursday, and "deal with current affairs such as the crisis facing cities and terrorism."
Before Trump takes to the stage, his daughter Ivanka will try to warm up the crowd and soften her father's image.
In a slew of emails to supporters Thursday, she spoke of a loving dad who encouraged his young daughter to succeed, of an inspirational leader and a crack negotiator destined to win.
"My father is someone you want fighting for you," she wrote. "He will outwork everyone in the room. He will always stay one step ahead of his competitors."
Trump's roller-coaster campaign defeated 16 rivals and steamrolled stubborn party opposition after being written off as a joke.
He has shocked foreign leaders by questioning key pillars of American foreign policy.
On Wednesday he qualified normally sacrosanct support for NATO allies, warning it would depend "if they fulfill their commitments to us."
Responding to the comments, NATO chief Jens Stoltenberg told AFP there was a need for solidarity.
"I will not interfere in the US election campaign, but what I can do is say what matters for NATO," he said.
"Solidarity among allies is a key value for NATO. This is good for European security and good for US security. We defend one another."
But Trump's biggest problem may be among Republicans. Many establishment Republicans do not support him and the convention has seen a key leader in the party's conservative wing break ranks.
On Wednesday, the rapturous welcome for arch conservative Senator Cruz turned into deafening boos after he provocatively told delegates to "vote your conscience" in November.
Eric Trump, the nominee's second son, described the speech as "classless."
But Cruz was unrepentant, defending himself at a breakfast meeting with Republicans from his home state of Texas.
"We're not going to win this election by yelling and screaming and attacking people," he said to applause.
Trump and Cruz were at loggerheads on the primary campaign trail: Cruz complaining that Trump was not a proper conservative and about his allegedly liberal "New York values" while the tycoon savaged Cruz as "Lyin' Ted" and posted a deeply unflattering photograph of his banker wife, Heidi.
Cruz had at one point pledged to support the eventual nominee, but he was defiant Thursday: "That pledge was not a blanket commitment that if you go and slander and attack Heidi, that I'm going to nonetheless come like a puppy dog and say thank you very much for maligning my wife."
It was left to Trump's pick for vice president, the socially conservative Indiana Governor Mike Pence, to try to overcome the Cruz debacle in delivering a speech introducing himself to voters.
He fed the crowd self-deprecating jokes and a clear conservative message, defending Trump as a man "who never quits, who never backs down" in a message given a standing ovation.
The most unifying aspect of the convention has been savage assaults on Clinton, portraying her as a criminal and a liar who should be jailed, with cries of "lock her up, lock her up."
Clinton, who will formally accept the Democratic nomination at her own convention next week, is expected to steal the limelight on Friday or Saturday by announcing her vice presidential running mate.