GETTY Donald Trump is the Republican nominee for this year's presidential election

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Mr Trump announced his intention to run for President a year ago, having never held political office before. The billionaire property mogul captured the imagination of voters by positioning himself as the anti-establishment vote. The Republican Party officially confirmed Mr Trump as their nominee for the upcoming elections at last month’s party convention to the shock of the world.

But Mr Trump’s attack on the grieving family of a slain Muslim American soldier and his refusal to endorse Paul Ryan and John McCain for re-election has generated a backlash within the Republican Party. US President Barack Obama has slammed Mr Trump as “unfit” for America’s highest office and called on Republican leaders to withdraw support for him. Rumours are now spreading that the billionaire could drop out before November's election.

What will happen if Trump quits? If Mr Trump were to quit the race at this stage, the fallout would set a precedent in US politics. What happens next would depend on timing. Despite President Obama's calls for the Republican Party to drop their support for Mr Trump, officials say they would not be able to remove him at this point, meaning he would have to quit voluntarily. Should he do so, the RNC would choose a replacement candidate, which could be Mr Trump's former opponent Ted Cruz, Mr Ryan or another member of the party. But Mr Trump would have to drop out by early September to give the party time to choose a replacement ahead of the November election. "If he were to drop out in the coming weeks, it would give the Republican party a chance to wrestle with and remedy the problem, according to its own rules," historian Alexander Keyssar told Express.co.uk. "What I don’t know is whether there are varying state laws about deadlines after which you can’t take a name off the ballot."

GETTY President Obama has condemned Mr Trump

Mr Keyssar, a historian at Harvard University, says that things would be less straightforward if Mr Trump waits until after the election to quit. The US relies on a complicated system called the Electoral College to deliver a new President. Although US citizens will cast their vote in November, the College technically chooses the new President when they meet in December. If Mr Trump quits after the popular vote but before the Electoral College meets, Mr Keyssar said the College could be forced to elect a new President among themselves. Failing that, the responsibility could fall to the House of Representatives with the Senate deciding on a Vice President. Following the Electoral College, the new President will be inaugurated in January. If Mr Trump waited until then to quit, the presidency would likely be passed on to the Vice President – his running mate Mike Pence.

GETTY Mr Trump clinched the nomination despite never having held office

Which Republicans are not supporting Trump?

Despite winning the party’s nomination, many Republicans are not supporting Mr Trump’s bid for the presidency. Mr Cruz has refused to endorse his former adversary after a bitter primary battle. Speaking at the Republican National Convention (RNC), he said the US "deserves leaders who stand for principle, unite us all behind shared values and cast aside anger for love". Paul Ryan, Speaker of the House, made a speech backing Mr Trump for President at the RNC. He said: “Only with Donald Trump and Mike Pence do we have a chance at a better way.” Mr Ryan and Mr McCain are among Republicans who have condemned Mr Trump’s attack on Khizr and Ghazala Khan, the parents of a Muslim soldier who died serving in Iraq. In retaliation, Mr Trump refused to back Mr Ryan's bid for re-election as House Speaker. He also refused to endorse Senator John McCain for re-election.

Donald Trump's most bizarre and uncomfortably awkward quotes Tue, November 8, 2016 Donald Trump business mogul turned political candidate is well known for his off the wall and often offensive quotes; here are some of his most mind boggling quotes from over the years. Play slideshow AFP/Getty Images 1 of 31 A recording of Donald Trump emerged during his campaign of him boasting back in 2005 “I’m automatically attracted to beautiful — I just start kissing them. It’s like a magnet. Just kiss. I don’t even wait. And when you’re a star, they let you do it. You can do anything.” He then went on to say “Grab ’em by the pussy. You can do anything.”

Mr McCain, who ran for President in 2008, said: ”I cannot emphasise enough how deeply I disagree with Mr Trump's statement. I hope Americans understand that the remarks do not represent the views of our Republican Party, its officers, or candidates." Another former Presidential hopeful, Mitt Romney, has been vocal opponent of Mr Trump. The 2012 Republican candidate said that a Trump presidency could usher in an age of “trickle-down racism”. He said: "I don't want to see a President of the United States saying things which change the character of the generations of Americans that are following. Presidents have an impact on the nature of our nation, and trickle-down racism, trickle-down bigotry, trickle-down misogyny, all these things are extraordinarily dangerous to the heart and character of America." Former Presidents George W Bush and his father George HW Bush were both noticeably absent from the RNC. Without naming names, Bush junior criticised Mr Trump’s policies of “isolationism, nativism and protectionism” at a charity event on Tuesday. His younger brother, Jeb Bush, notably ran against Mr Trump for the Republican nomination and has also refused to offer his endorsement. Sally Bradshaw, Jeb Bush’s top advisor, yesterday announced that she was leaving the party, saying she would likely vote for Hillary Clinton in November’s election.

GETTY Ted Cruz has refused to endorse Mr Trump