“If we win Indiana, it’s over,” Trump told a cheering crowd in Terre Haute, Indiana.
The blunt-spoken real estate mogul holds a double-digit polling lead over US senator Ted Cruz of Texas, who has been campaigning in the midwestern state almost non-stop since mid-April.
Cruz has trumpeted Indiana, one of the last big states left in the fight to get onto the November 8 presidential ballot, as his golden moment to stop Trump and force a brokered nomination at the party’s July convention.
But it appeared to be shaping up as his Waterloo.
Fresh off a sweep of five northeastern states last week, Trump knows a win in Indiana will put him within reach of the 1,237 delegates required to lock up the Republican nomination before the convention.
Top Trump aide Corey Lewandowski told CNN on Tuesday the campaign expected to win more than the required number of delegates — 1,300 to 1,400.
Cruz vowed to “compete to the end” but a loss in Indiana would be particularly crushing for the senator, who has argued that his brand of religious conservatism is a natural draw for heartland Republicans.
He won the endorsement of conservative Indiana governor Mike Pence.
Cruz had hoped for smoother sailing in Indiana after he and John Kasich reached a “stop-Trump” deal in which Kasich would steer clear of the state while Cruz would do likewise in Oregon and New Mexico.
But the waters are looking choppier for Cruz, with the senator losing considerable ground against Trump in opinion polls as voting neared.
Cruz last week also announced his choice for a prospective vice president, the former presidential contender and Hewlett Packard chief executive Carly Fiorina, during an event in Indiana that some criticised as premature.
Trump has drawn both passionate support and vitriolic condemnation with his stands on immigration and national security — including a call to build a 1,000-mile wall along the Mexican border that he says Mexico would pay for and a bid to temporarily ban Muslims from entering the country.
Julie Blackwell Chase, a clerk treasurer in the town of Bedford in southern Indiana, said she voted early for Trump in part because she appreciated his willingness to break with conventional politics.
“We need new blood,” she said.
The outcome in Indiana may ride on the votes of evangelical Christians after Trump offered praise for planned parenthood family clinics and signalled support for gay and transgender rights — views that rankled some Christian conservatives.
Donald Trump repeats unsubstantiated claims that Ted Cruz's dad has links to JFK's assassin, Lee Harvey Oswald. https://t.co/5c87fTEuBl— The Associated Press (@AP) May 3, 2016