A new NBC/Wall Street Journal/Marist opinion poll showed Trump with a wide lead in Indiana, 49 percent to 34 percent for Cruz and 13 percent for a third candidate, Ohio Governor John Kasich.

Trump, a 69-year-old billionaire real estate developer, sounded confident in an interview on "Fox News Sunday" 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." 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 U.S. Secretary of State Hillary Clinton, 68, leads US Senator Bernie Sanders, 74, of Vermont in the race for the Democratic nomination.

On NBC's "Meet the Press," Cruz, 45, was asked several times whether he would support Trump if the New York businessman was the Republican nominee. Cruz evaded the question each time and turned the questions into an attack on broadcast media.

I recognize that many in the media would love to see me surrender to Donald Trump because that means that Hillary wins. The media has given USD 2 billion in free advertising to Donald Trump," Cruz said.

Cruz said he has momentum in Indiana based on his choice of former candidate Carly Fiorina for his vice president and Friday's endorsement by Indiana Governor Mike Pence. Americans will elect a successor to President Barack Obama on November 8.

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 are awarded from each of the state's nine U.S. congressional districts with the candidate who receives the most votes taking them all. The 30 others are awarded to the candidate who wins the most votes statewide.

At a rally in Terre Haute, Indiana, Trump urged Republicans to join his "movement" and turn out for him in big numbers.

"The more we can win by in Indiana is so important. Its a mandate ... a really important mandate. It’s a mandate for change, but not Obama change. Real change. Its a mandate for genius," he said.

US Senator Lindsey Graham of South Carolina, a leading Republican critic of Trump, called him the "most unelectable person" the party could nominate. Graham had sought the nomination himself.

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