Donald Trump is now the presumptive Republican presidential nominee after Ted Cruz dropped out of the race. Bernie Sanders scored a big upset over Hillary Clinton in the Indiana primary. Photo: Associated Press

DONALD Trump’s stunning victory in Indiana on Tuesday and rival Ted Cruz’s abrupt decision to drop out of the race have plunged the Republican Party into turmoil.

Once dismissed as a fringe candidate, Trump is now certain to be the Republican presidential nominee. With Cruz gone and John Kasich - the awkward third wheel of the campaign - about to quit as well, Trump will be running unopposed in the remaining primaries.

That means the real estate mogul is guaranteed to secure the 1237 delegates he needs to win the nomination.

Trump is now bracing for a general election fight against the presumptive Democratic nominee, Hillary Clinton. But his first challenge is to unite the party his candidacy has left bitterly divided.

Some Republican leaders are gradually warming to the former Apprentice host, even though many of them warned he was “unfit” to be president mere months ago. Take Louisiana Governor Bobby Jindal, who once described the real estate mogul as a “dangerous egomaniac”, but now says he’ll vote for him.

Not everyone is so willing to bury the hatchet. Other prominent conservatives see Trump as an existential threat to their fractured party, and are now considering the ultimate betrayal: voting against their own party.

In the 24 hours since Trump’s latest victory, these Republicans have raced to distance themselves from him, declaring their opposition in a flurry of social media posts and interviews.

Despite a call for unity from Republican National Committee Chairman Reince Priebus, at least three shellshocked Republican senators — Lindsey Graham of South Carolina, Nebraska’s Ben Sasse and Cory Gardner of Colorado — have already reiterated their opposition to Trump in no uncertain terms.

If we nominate Trump, we will get destroyed.......and we will deserve it. — Lindsey Graham (@LindseyGrahamSC) May 3, 2016

Reporters keep asking if Indiana changes anything for me. The answer is simple: No. This from Febr. still holds:https://t.co/yUNSZTHW7E — Ben Sasse (@BenSasse) May 4, 2016

Rory Cooper, a senior adviser to the #NeverTrump movement, said the group would keep up the fight in an effort to save Republican senate and congressional candidates from electoral oblivion.

“A strong Never Trump movement is critical to protecting Republican incumbents and down-ballot candidates, by distinguishing their values and principles from that of Trump, and protecting them from a wave election,” he said in a statement.

“Never does not mean maybe. Six out of 10 Republican primary voters voted for someone other than Trump, and we will continue to identify ways to give them voice.”

Katie Packer, a former aide to 2012 presidential nominee Mitt Romney who is now chair of the anti-Trump Our Principles PAC, said in a statement that her group would continue making the case that Trump is “simply unfit to be President of the United States”.

“If we nominate Trump, [the party] is lost beyond this cycle. I think we lose women for a generation, in big numbers ... So there’s a sense that if this is who my party is, I don’t really identify with it anymore,” Packer added.

Romney himself, as a key leader of the party’s establishment wing, has more power than most to help Trump’s push for unity, but he’s shown no inclination to do so. In fact, in a speech several months ago, he called Trump a “phony and a fraud” who was “playing the American public for suckers”.

Some other conservatives who clearly share those views have taken to Twitter to post videos of themselves burning their Republican voter registration cards.

John McCain’s former campaign speechwriter Mark Salter and conservative pundit Ben Howe both went a step further, signalling their support for Clinton on Tuesday night.

the GOP is going to nominate for President a guy who reads the National Enquirer and thinks it's on the level. I'm with her. — Mark Salter (@MarkSalter55) May 3, 2016

I am no longer a Republican. — Ben Howe (@BenHowe) May 4, 2016

It’s difficult to determine exactly how many Republicans intend to abandon their party in the general election, but a new poll gives us a clue. It shows less than a third of Ted Cruz’s voters have a favourable opinion of Trump, and 13 per cent already admit they would back Clinton over him.

This is important for Trump. Republican and Democratic nominees are generally supported by more than 90 per cent of their own parties’ voters in presidential elections, and if that number slips at all, it can be fatal to their chances of victory.

So, Trump will either need to attract millions of new voters from outside the party, or win over millions of anti-Trump Republicans by November.

Trump did extend an olive branch to his detractors on Tuesday night, offering them a seat on the “Trump train”.

“They are calling now,” he said. “And they are calling to say, ‘We’d love to get on the train.’ The Trump train. We have a lot of people coming.”

Clinton and Trump now plunge into a six-month battle for the presidency. Dominating talk shows in the US on Wednesday morning, Trump expressed confidence that he will beat Clinton when they get to the ballot box, despite trailing the former secretary of state 54-41 in a new CNN poll.

“I think we’re gonna beat Hillary Clinton,” he told Fox News. “She’s horrible on the economy, horrible on jobs. She’s really horrible on the military.”

As Trump’s rise has already demonstrated, anything is possible. But the anti-Trump forces within the Republican party, together with the full force of the Clinton machine, will give him a very steep hill to climb.