Front-runner Donald Trump says 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, a 69-year-old real estate developer, sounded confident in an interview on Fox News when asked whether Indiana would basically end the long-running Republican race in his favor.
"Yes, it's over," Trump said.
"It's already over."
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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 1237 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, 68, leads US Senator Bernie Sanders, 74, 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.
Americans will elect a successor to President Barack Obama on November 8.
