Donald Trump has taken a major step towards sewing up the Republican presidential nomination with a victory in Indiana's primary election, dashing the hopes of rival Ted Cruz.
Hillary Clinton and Bernie Sanders were vying for victory in the Democratic primary, though it was too early to call the race as votes were being tallied. Clinton already is 91 percent of the way to her party's nomination.
While Trump can't mathematically clinch the GOP nomination with his victory in Indiana, his path now becomes easier and he has more room for error in the remaining primary contests.
Trump's win also was a big psychological blow to Cruz, the conservative Texas senator who hasn't topped Trump in a month.
Cruz campaigned vigorously in Indiana, securing the endorsement of the state's governor and announcing businesswoman Carly Fiorina as his running mate. But he appeared to lose momentum in the final days of campaigning and let his frustration with Trump boil over yesterday, calling the billionaire "amoral" and a "braggadocious, arrogant buffoon".
Trump, who frequently refers to Cruz as "Lyin' Ted", quickly responded to his rival's attack.
"Over the last week, I have watched Lyin' Ted become more and more unhinged as he is unable to react under the pressure and stress of losing, in all cases by landslides, the last six primary elections -- in fact, coming in last place in all but one of them," Trump said in a statement.
Cruz also termed Trump a "serial philanderer" -- likely as part of his strategy to try to siphon the support of evangelical voters from Trump.
Cruz has vowed to stay in the race through the final primaries in June, clinging to the possibility that Trump will fall short of the 1237 delegates he needs and the race will go to a contested convention.
But he could face pressure from donors and other Republicans to at least tone down his attacks on Trump in an attempt to unite the GOP heading into the general election.
Whether a united Republican Party is even possible with Trump at the helm remains deeply uncertain. Even before the Indiana results were finalised, some conservative leaders were planning a Wednesday meeting to assess the viability of launching a third party candidacy to compete with Trump.
Only about half of Indiana's Republican primary voters said they were excited or even optimistic about any of their remaining candidates becoming president, according to exit polls. Still, most said they probably would support whoever won for the GOP.
Clinton, too, needs to win over Sanders' enthusiastic supporters. The Vermont senator has cultivated a deeply loyal following in particular among young people, a group Democrats count on in the general election.
Sanders has conceded his strategy hinges on persuading superdelegates to back him over the former secretary of state.
Superdelegates are Democratic Party insiders who can support the candidate of their choice, regardless of how their states vote. And they favour Clinton by a nearly 18-1 margin.
Exit polls showed about seven in 10 Indiana Democrats said they'd be excited or at least optimistic about either a Clinton or Sanders presidency. Most said they would support either in November.
A showdown between Clinton and Trump would pit one of Democrats' most experienced political figures against a first-time candidate who is deeply divisive within his own party.
Cruz and other Republicans have argued that Trump would be roundly defeated in the general election, denying their party the White House for a third straight term.