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 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 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 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, 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 $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 Nov. 8.

'THE MOST UNELECTABLE PERSON'

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 US 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. It's a mandate ... a really important mandate. It's a mandate for change, but not Obama change. Real change. It's 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.

"Keep fighting Ted," Graham told CBS's "Face the Nation."

CLINTON URGES SANDERS QUIT

On the Democratic side, front-runner Clinton told CNN's "State of the Union" that rival Bernie Sanders has been "helpful" in bringing millions of people into the party's presidential race, but it was time for him to step aside.

"There comes a time when you have to look at the reality," said Clinton, who won four of the five Northeastern states that voted last Tuesday and who has a big lead in the delegate race ahead of the Democratic National Convention in Philadelphia on July 25-28.

But at a news conference in Washington, Sanders refused to get out, saying he believes the Democratic battle will end up in a contested convention.

Sanders said it was nearly impossible for Clinton to win the 2,383 delegates needed for nomination without superdelegates, who are unelected and free to support any candidate they wish.

"We intend to fight for every vote and delegate remaining," he said. Clinton has 2,165 to Sanders' 1,357 delegates, according to an AP count that includes superdelegates who have said whom they support.

In his Fox interview, Trump defended at length his views on foreign policy, which he outlined in a speech last week in Washington that drew criticism for sometimes contradictory views. Trump said he would move quickly to destroy Islamic State's militancy, but would resist interventionist policies in order to focus on nation-building at home.

Trump said "every move we made in the Middle East was wrong" over the past 15 years, with lives and money wasted. He said he would resist such policies.

Asked whether the United States should return to working with "strongmen" leaders like the late Iraqi President Saddam Hussein, Trump said: "Isn't it too bad that we knocked him out in the first place?"- Reuters