Republican Donald Trump says the party's leaders do not want him to win the presidential nomination.

The system is "stacked against me," he said at a town hall event in New York, and accused the Republican National Committee of conspiring to stop him.

His comments came after his rival Ted Cruz was awarded all the delegates in Colorado without a state-wide vote.

Trump leads the race but may fall short of getting enough delegates to get the nomination outright.

That would lead to a contested convention in July, where delegates are free after the first ballot to back whom they want, opening the door for Texas Senator Cruz or even the third candidate in the race, John Kasich.

The Washington Post reported on Wednesday that Cruz is likely to win on a second vote, because he has persuaded so many delegates to vote for him when they are "unbound" to vote as pledged.

But Republican National Committee chairman Reince Priebus rejected Trump's charge that the rules in states like Colorado were changed in response to his rise in the polls.

Priebus tweeted that the nomination process was well known for more than a year.

"It's the responsibility of the campaigns to understand it. Complaints now? Give us all a break."

Asked at the town hall whether the RNC wanted him to win, Trump said: "No, I don't think so. I really don't."

He has been criticised for not campaigning hard enough on the ground in states like Colorado.

But Trump said delegates who wanted to support him were pushed out by the RNC.

Most states have opted to hold state-wide primaries or caucuses to determine the number of delegates pledged to a particular candidate. Photo: AFP

"They don't like when I put up my own money because it means they don't have any control of me because I'm working for the people," he said.

Most states have opted to hold state-wide primaries or caucuses to determine the number of delegates pledged to a particular candidate.

But Colorado decided last summer to select its delegates in a different way, at its own state convention.

The state-by-state primary contests come to New York next week where a high number of delegates will be up for grabs.

Several senior Republicans have expressed opposition to Trump winning, doubting his ability to win a general election and disagreeing with his hard line on immigration.