The chairman of the US Republican National Committee has declared Donald Trump the party's "presumptive nominee" after the property mogul's resounding victory in Indiana.
The billionaire outsider 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".
.@realDonaldTrump will be presumptive @GOP nominee, we all need to unite and focus on defeating @HillaryClinton #NeverClinton
— Reince Priebus (@Reince) May 4, 2016
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.
Republican presidential hopeful Ted Cruz has suspended his White House campaign after being soundly defeated in Indiana.
The Texas senator, who previously vowed to fight on to the party's convention, shocked supporters in Indianapolis by announcing his exit.
"We left it all on the field in Indiana," Mr Cruz said. "We gave it everything we've got but the voters chose another path".
"With a heavy heart, but with boundless optimism for the long term future of our nation, we are suspending our campaign".
Ted Cruz: "It is with a heavy heart... we are suspending our campaign"https://t.co/eE8QS1uM7Z
— Mashable News (@MashableNews) May 4, 2016
The announcement came on the heels of Republican front runner Donald Trump's emphatic win in the Hoosier State primary.
The Cruz campaign had high hopes for Indiana, particularly after he and GOP rival John Kasich formed an unlikely alliance in a last-gasp bid to stop Mr Trump.
Tensions were clearly high heading into Tuesday as Mr Cruz and Mr Trump engaged in their latest volley of harsh personal attacks.
But following a string of defeats in the northeast, and with the billionaire businessman poised to take all of Indiana's 57 delegates, the writing was on the wall for the conservative firebrand.
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."
Ted Cruz is "one hell of a competitor" says Donald Trump after Texas senator drops out https://t.co/y0MCZKsQH0 https://t.co/DsQ8MYNxyG
— BBC News (World) (@BBCWorld) May 4, 2016
Not even the late addition of former technology executive Carly Fiorina to the Cruz ticket was enough to overcome Mr Trump's growing lead.
Mr Cruz rode into Washington on a wave of tea party support in 2010, and quickly established himself as a thorn in the side of both Democrats and Republicans.
Just last week, former Republican Speaker of the House John Boehner referred to Mr Cruz as "Lucifer in the flesh".
Although the junior senator's White House bid was viewed as unlikely, he received a massive boost when he won the all-important Iowa caucuses in February with the support of the state's large evangelical population.
He would go on to win eight more states in the months to follow, with his last victory coming in Wisconsin on April 5th.
Mr Cruz still holds his US Senate seat for another two years before facing re-election in 2018.
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.
