Donald Trump led by 15 points in latest polls for the Indiana primary, where rival Ted Cruz has mounted a last-ditch effort to stop the outspoken frontrunner from sewing up the Republican nomination, while the Democratic aspirants Hillary Clinton and Bernie Sanders were nearly tied.



A win in the primary, scheduled for tomorrow, will bring as many as 57 delegates for Trump and put him in a commanding position, making it difficult for Cruz to pose any serious challenge to the real-estate tycoon before the July Republican convention.



The latest NBC-Wall Street Journal poll said that Trump has the support of 49 per cent likely voters and Cruz 34 per cent, followed by Ohio Governor John Kasich at 13 per cent.



Trump and Cruz are blitzing the crucial state. Trump has held a number of rallies there and has also blasted Cruz in TV ads as supporting "bad trade deals [that] have hurt Indiana."



The poll indicated a majority of Republican voters opposed a pact between Cruz and Kasich by 58 per cent to 34 per cent.



Republican voters said they disagreed with the decision by Cruz and Kasich to compete in different states in an attempt to deny Trump the first-ballot victory. As part of the deal, Kasich is not campaigning in Indiana.



"Indiana is an important state," Cruz said yesterday on NBC's 'Meet the Press.'



Trump, who has declared himself the "presumptive nominee", said the contest would end if he won the state. "I think it's over now, but it's over," Trump told Fox News.



"Cruz cannot win, he's got no highway, he's got nothing, he's way behind. I'm leading him by millions and millions of votes and I'm leading him by 400 or 500 delegates."



But Cruz, who is already eliminated from reaching 1,237 delegates needed to win the nomination outright, said he would prevent Trump from a first-ballot victory. He desperately needs a victory in Indiana to attempt to clinch the nomination at the convention in Cleveland this summer.



"We are headed to a contested convention, and we're gonna win, and I'm not willing to concede this country," he said on NBC.



Among the Democrats, Clinton, with 50 per cent supporters, leads Sanders by 4 per cent. Sanders said yesterday his path to nomination is difficult, but not impossible.



Sanders has told his supporters he is not going anywhere and that he too is counting on a contested convention.



"Let me be very clear. It is virtually impossible for Secretary Clinton to reach the majority of convention delegates by June 14 with pledged delegates alone. In other words, the convention will be a contested contest," he said.



But Clinton already seems to be wooing Sanders supporters by releasing an ad that encourages Democrats to come together.



"America is stronger when we are all supporting one another," she has said in a campaign ad.