Donald Trump is carrying a commanding lead into Tuesday's potentially decisive Indiana Republican primary, according to a new poll that gives him a 15-point edge.

A near majority of Hoosier Republicans, 49 per cent, back the business billionaire Trump. Texas Sen. Ted Cruz is trailing with 34 per cent. Ohio Gov. John Kasich is far behind with 13 per cent.

The NBC News/Wall Street Journal survey of likely GOP voters, conducted by Marist University's Institute for Public Opinion, suggests a ringing Trump endorsement from basketball coaching legend Bobby Knight has moved the needle significantly.

A CBS News/YouGov poll published seven days ago had Trump lead hovering at just 5 percentage points. Just one week before that, Cruz led Trump by 16 points in a poll conducted by Indiana University – Purdue University Fort Wayne.

'We were doing really, really well in Indiana,' Trump told a crowd in Terre Haute on Sunday, 'but then I brought out Bobby Knight, and look at these polls! Like a rocket! It's like a rocket ship.'

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WINNING BY THIS MUCH: Donald Trump holds a 15-point lead in the winner-take-all state of Indiana just two days before a critical primary election there

PUT ME IN, COACH: Legendary Indiana hoops coach Bobby Knight, a cult hero in Hoosier Nation, endorsed Trump loudly and repeatedly last week

YUUGE LEAD: Trump has a 15-point cushion in Indiana, but even the slimmest of victories would give him a sweep of the state's 57 convention delegates

Cruz is now campaigning hard to win back some of those votes in a state he admits will be crucial if he is to make a viable case for taking the Republican nomination.

The Terxas senator told ABC: 'We are competing hard. I hope we do well here. I can tell you I'm barnstorming the state, we're in a bus with my family, we're doing everything we can to earn the votes of the men and women in this state.

'We're going the distance. We're competing the entire distance.'

Cruz said he was thankful the primary would be decided by 'Midwestern common sense' and touted his own endorsement from governor Mike Pence, who he called a 'common sense conservative.

However, Cruz stopped short of calling the state a must-win, and instead called it 'critical', leaving the door open for his candidacy to continue even if he looses overall.

Knight backed Trump in a series of rallies and broadcasts last week, praising him in colorful language as the best-prepared man to be president in the history of the United States.

Cruz's public deal with Kasich, by contrast, appears to be a dud.

Kasich agreed not to campaign in Indiana in exchange for Cruz returning the favor in Oregon and New Mexico. the two campaigns announced the strategy with simultaneous press releases a week ago.

But Kasich reneged almost immediately, telling reporters – despite Cruz's claim he had 'pulled out' of the state – that he still wanted Indiana Republicans to vote for him.

The political carve-up, designed to orchestrate one-on-one fights between Trump and his remaining rivals, arrangement earned a thumbs-down from Indianans in the NBC/Wall Street Journal poll.

Fifty-eight per cent of those surveyed said they don't like the deal, compared with just 34 per cent who do. That 24-point margin of disapproval is larger than the 15-point spread Trump enjoyed overall.

ON LIFE SUPPORT? Texas Sen. Ted Cruz's chance of stopping Trump's nomination march will slim down considerably if he can't pull a rabbit out of a hat in Indiana on Tuesday

THE ART OF THE NON-DEAL: Ohio Gov. John Kasich had agreed to stay out of Indiana and clear the way for a head-to-head matchup betwen Trump and Cruz, but he insisted people should vote for him anyway

It's still theoretically possible for Cruz to prevent Trump from collecting the 1,237 Republican National Convention delegates he would need to clinch the nomination, but stopping him would become the longest of long shots if Trump wins Indiana's primary election on Tuesday.

If he prevails, Trump would make a clean sweep of the state's 57 delegates, adding them to a total that news organizations estimate at anywhere from 991 to 998.

Trump insisted on Friday that he had crossed into quadruple-digits with 1,001.

Anti-Trump Republican factions hope the real estate tycoon can be stalled just short of the finish line, throwing July's nominating convention into the chaos of a floor fight.

But most Indiana Republicans believe that instead of an arm-twisting exercise behind the scenes, the attention in such a circumstance should shift to the number of votes the candidates received during the primaries.

Nearly two-thirds told pollsters that the man with the most votes should win the nomination if no one can amass the support of 1,237 delegates.

That would favor Trump, with nearly 10.1 million votes, by a wide margin. He leads Cruz by more than 3.2 million.