The Republican front-runner has scored another key victory over rival Ted Cruz, according to projections by US TV networks. Hillary Clinton and Bernie Sanders were reported to be too close to call in the Democratic vote.
Trump was on track to take well over 50 percent of the vote, US broadcasters announced on Tuesday, bringing him much closer to outright victory in the race to be the party's candidate for the White House.
"Thank you Indiana!" Trump tweeted as the vote was called in his favor.
Cruz had been counting on a win in Indiana's primary to slow the New York billionaire's progress toward the Republican nomination.
But polls in recent days showed Trump opening up a substantial lead in the Midwestern state over the Texas senator, whose brand of Christian conservatism had been expected to have wide appeal.
Based on 13 percent of precincts reporting, the billionaire took 53.8 percent of the vote, streets ahead of Cruz on 33.9 and Ohio Governor John Kasich on 9.4 percent.
Trump strides ahead
To win the nomination, Trump needs support from 1,237 Republican delegates and has currently amassed 1,002 names, according to CNN's tally. Cruz is at 572 delegates, while Kasich trails with 156.
If Trump sweeps Indiana's 57 delegates, the Cruz camp has admitted it faces an extremely steep road ahead.
The figures appeared to confirm the failure of a so-called "Stop Trump" movement against the outspoken businessman, which had grown louder in recent weeks.
Ahead of Tuesday's vote, the attacks between the two rivals grew noticeably more personal. In a Fox News interview, Trump linked Cruz's father to the murderer of John F. Kennedy, causing the Texas senator to fire back by calling Trump a "moron," among other things.
"We are not a proud, boastful, self-centered, mean-spirited, hateful, bullying nation," Cruz said alongside his wife Heidi and running mate Carly Fiorina earlier in the day. "If Indiana does not act, this country could well plunge into the abyss."
Trump had his own words for Cruz, however. In a statement released following the senator's most recent comments, Trump said Cruz was "a desperate candidate trying to save his failing campaign."
"Today's ridiculous outburst only proves what I have been saying for a long time, that Ted Cruz does not have the temperament to be President of the United States."
Democratic race tight
The Democratic race between Hillary Clinton and Bernie Sanders was still too early to call in the Midwestern state, but early voting results showed the former secretary of state with a razor-thin lead against her rival.
Wins by both Trump and Clinton in Indiana would shore up the chances of the pair locking horns ahead of the November elections, analysts said.
mm/rc (AFP, Reuters)