The chairman of the Republican National Committee has declared Donald Trump the party’s “presumptive nominee” after the property mogul’s resounding victory in Indiana.
The billionaire businessman was projected to grab more than 50% of the vote in the crucial Midwestern clash with 95% of the precincts reporting.
The drubbing prompted Mr Trump’s chief rival Ted Cruz to announce he was ending his campaign – leaving Ohio Governor John Kasich as the businessman’s only remaining challenger.
On the Democratic side, Hillary Clinton was narrowly defeated by rival Bernie Sanders, who vowed to fight on despite being mathematically out of the race.
Mr Trump’s win, meanwhile, was widely considered the death knell for Mr Cruz and Mr Kasich after their last-gasp alliance to thwart the front runner’s hopes failed to yield results.
Both were mathematically eliminated last month from reaching the 1,237 delegates needed to secure the GOP nomination.
Following Mr Cruz’s departure, RNC chairman Reince Priebus declared Mr Trump the party’s “presumptive nominee” on Twitter.
He urged Republican voters to “unite and focus on defeating Hillary Clinton”.
:: Donald Trump: What’s Next After Indiana Win?
Mr Kasich’s campaign issued a statement on Tuesday night saying he will remain in the race unless a candidate locks up the nomination before the convention in July.
Indiana’s Blackford County Republican Party Chairman Jack Beckley told Sky News it was time for Mr Kasich “to admit defeat”.
“The people have made it very clear that they are sick and tired of politicians and business as usual in Washington DC,” he said.
“The people have spoken. We need to unite.”
The latest victory will put Mr Trump over the 1,000-delegate mark, with high stakes contests still on the calendar, including California next month.
Following weeks of rising tension between the Trump and Cruz camps, the New York business magnate softened his tone on the conservative Texas senator.
During his victory speech, Mr Trump called Mr Cruz “one hell of a competitor”.
“He is a tough, smart guy, and he has got an amazing future. I want to congratulate Ted. I know how tough it is.”
For Mrs Clinton, the defeat to Mr Sanders in Indiana does very little to block the former secretary of state’s path to the nomination.
At this stage of the campaign, Mr Sanders has no chance of reaching the 2,383 delegates needed to win, but the self-described Democratic socialist has vowed to continue his campaign.
“The Clinton campaign thinks this is over. They’re wrong,” he told the Associated Press news agency.