Cleveland - Donald Trump takes centre stage to deliver the speech of his political life on Thursday, trying to salvage a fractious Republican convention that saw a key rival showered in boos for withholding his endorsement.Appearing before delegates, the most unorthodox and controversial presidential contender in modern times will accept the nomination of a party that has guided more candidates to the Oval Office than any other.But instead of sounding the starting gun on the general election and placing his candidacy in the sweep of American history, Trump faces a much more basic task.Before a television audience of tens of millions, the bombastic billionaire will have to prove to the American people that he is worthy of the White House and capable of being commander-in-chief.The four-day nominating convention, which ends in Cleveland on Thursday, has been plagued with deep party divisions and staggering campaign blunders.'Vote your conscience' What began on Monday with anti-Trump delegates voicing misgivings on the convention floor, culminated in a moment of high political drama on the eve of Trump's speech.Trump's primary rival Ted Cruz threw the convention into chaos by provocatively telling delegates to "vote your conscience" in November.Ovations and adulations for the conservative Texas senator turned to jeers and boos as it became clear he had not come to Cleveland to endorse the party nominee."We deserve leaders who stand for principle, who unite us all behind shared values, who cast aside anger for love," said Cruz, effectively listing criticisms often levelled at Trump.In another dramatic twist, Trump decided to step on Cruz's moment in the sun by entering the arena before the Texan had finished speaking.Cruz - who is widely expected to run in 2020 should Trump lose to Hillary Clinton - was forced to wave and smile before completing his remarks and exiting to more jeers."Wow, Ted Cruz got booed off the stage, didn't honour the pledge!" Trump tweeted later, referring to a pledge by Republican primary candidates to support whoever won the party's nomination.The two men fought a bitter and at times deeply personal battle for the nomination, in which Trump mocked Cruz's wife and tarred his rival with a nasty nickname: "Lyin' Ted."Scandal and strifeBetween the uncharacteristic public spasms of Republican disunity came the embarrassing revelation that a prime-time speech by Trump's wife Melania had plagiarised remarks made by First Lady Michelle Obama.This was not how the convention was meant to go.In naming Indiana Governor Mike Pence as his running mate, Trump had tried to smooth difficult relations with conservatives and unite the party.Trump's caustic rhetoric on race, women and Islam had alienated many Republicans, to say nothing of swing voters.Pence did his bit, wooing the crowd with self-deprecating jokes and a clear conservative message in an acceptance speech.Trump, he joked, is "a man known for a large personality, a colourful style and lots of charisma and I guess he was just looking for some balance on the ticket."The 57-year-old said he "joined this campaign in a heartbeat" because Republicans nominated Trump, a man "who never quits, who never backs down."Throughout the convention, Trump's family members have also tried to soften his brash public image.Meanwhile, a string of Republican speakers savaged his Democratic opponent, describing Clinton as a liar who should be jailed.Trump's roller-coaster campaign defeated 16 rivals and steamrolled stubborn party opposition after being written off as a joke, the real estate tycoon having never held elected office.His campaign defied political norms - embracing racially inflammatory policies, offending key voting blocs, eschewing big-spending advertising campaigns and relying on saturated media coverage above campaign structure.In an interview with the New York Times on Wednesday, he raised anew questions about whether, as president, he would come to a Nato ally's defence if it were attacked, saying it would depend "if they fulfil their commitments to us."Trump told the Times he would press the theme of "America First" in his acceptance speech on Thursday night.Polls following the convention will show whether Trump can continue to defy political gravity.Many candidates have seen a bump in the polls following the party convention.Democrats, who will hold their own convention from Monday, will do their utmost to make sure Trump enjoys no post-nomination bounce.Clinton is expected to steal the limelight on Friday or Saturday by announcing her vice presidential running mate.