Washington: Front-runner Donald Trump said on Sunday that he will have essentially sealed the Republican US presidential nomination if he wins Tuesday's contest in Indiana, where he now holds a big lead over chief rival Ted Cruz.
A new NBC/Wall Street Journal/Marist opinion poll showed Trump with a wide lead in Indiana, 49 per cent to 34 per cent for Cruz and 13 per cent for a third candidate, Ohio Governor John Kasich.
Trump sounded confident in an interview on "Fox News Sunday" when asked whether Indiana would basically end the long-running Republican race in his favour.
"Yes, it's over," Trump said. "It's already over."
The poll showed the depth of the challenge facing Cruz, a conservative US senator from Texas who is trying to prevent Trump from winning the 1,237 delegates needed to seal the nomination.
Cruz's hopes rest on emerging as a consensus alternative to Trump at the Republican National Convention in Cleveland on July 18-21. Former US secretary of state Hillary Clinton leads US Senator Bernie Sanders of Vermont in the race for the Democratic nomination.
Trump, who has amassed 996 delegates, according to an Associated Press count, has momentum behind him and looks increasingly likely to win the nomination outright, without a contested convention, perhaps when California votes on June 7.
Indiana has 57 Republican delegates. Three delegates are awarded from each of the state's nine US Congressional districts with the candidate who receives the most votes taking them all. The 30 other delegates will be awarded to the candidate who wins the most votes statewide.
On NBC's "Meet the Press," Cruz said he has momentum in Indiana based on his choice of former candidate Carly Fiorina as his choice for vice president and Friday's endorsement by Indiana Governor Mike Pence.
"I think the support we are seeing is surging," Cruz said.
Cruz was pressed on whether he would support Trump if the New York billionaire is the Republican nominee. Cruz evaded the question each time and turned the questions into an attack on broadcast media.
"I recognise that many in the media would love to see me surrender to Donald Trump because that means that Hillary wins. The media has given $2 billion in free advertising to Donald Trump," Cruz said.
Americans will elect a successor to President Barack Obama on November 8.