Republican presidential candidate Donald Trump has lashed out, saying Colorado's votes could be bought, after his rival Ted Cruz won all 34 of Colorado's delegates.
Mr Trump lashed out at what he called the party's "rigged" delegate selection rules on Monday.
"There was no voting. I didn't go out there to make a speech or anything, there's no voting," Mr Trump said.
"The people out there are going crazy, in the Denver area and Colorado itself, and they're going absolutely crazy because they weren't given a vote. This was given by politicians - it's a crooked deal."
Texas senator Ted Cruz, won all 34 of Colorado's delegates.
In the modern political era, 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 to select its delegates in a different way, at its own state convention.
"Now they're trying to pick off those delegates one by one," Mr Trump said.
"That's not the way democracy is supposed to work. They offer them trips, they offer them all sorts of things and you're allowed to do that. You can buy all these votes."
Guy Short, a Cruz backer in Colorado who was elected as a Republican national convention delegate for the sixth time, disputed Mr Trump's allegations.
"Donald Trump is a liar," Mr Short told Reuters in an email.
"Nobody was offered anything. In fact, I spent thousands of dollars of my own money campaigning to become a delegate because it's that important to make sure Donald Trump is NOT our nominee."
Mr Trump told a rally on Sunday that the person who wins the most votes in the primary process should automatically be the nominee.
"What they're trying to do is subvert the movement with crooked shenanigans," he said.
The race for 1237
Mr Trump is still well ahead in the Republican race but if he does not get 1237 delegates in the state-by-state contests, he could lose out anyway.
A winner could well emerge from a contested convention in July.
In that scenario, many of the delegates could back who they want to after the first ballot, opening the door for Texas Senator Mr Cruz or even the third candidate in the race, John Kasich.
Mr Trump has 743 bound delegates and Mr Cruz has 545, according to an Associated Press count.
The Cruz campaign did not immediately respond to Mr Trump's Colorado accusations but spokeswoman Catherine Frazier said on Sunday the allegations were "more sour grapes".
"We are winning because we've put in the hard work to build a superior organization," she told CNN.
Senior Republicans voiced concern about Mr Trump's chances of winning in November's general election, and opposed his hardline views on immigration.
Mr Trump has recently brought in veteran strategist Paul Manafort to lead his delegate-acquisition efforts.
Mr Manafort said on Sunday Mr Cruz's campaign was using "Gestapo tactics" to win over delegates.
Mr Trump on Monday accused Mr Cruz of trying to steal delegates in South Carolina, where Mr Trump won the state primary in February.
Mr Cruz came in third in the South Carolina primary but won three delegates on Saturday at congressional district meetings, according to local media.
New York, New York
Next week, New Yorkers get the chance to pick their nominees in the Republican and Democratic contests, in a pivotal moment in the process.
Delegates are party members with the power to vote for each candidate at the party conventions held in July, where the nominee is formally confirmed.
Mr Trump also revealed that only one of his three children residing in the state of New York would be able to vote.
"They didn't register in time," he said. "So they feel very, very guilty."
Meanwhile, attention in the Democratic primaries campaign is turning towards New York as well.
Vermont Senator Bernie Sanders can now boast he has won seven of the latest eight states, however rival Hillary Clinton maintains a clear lead in the overall race.
If he can defeat Mrs Clinton in New York on 19 April - the state where she served as senator for six years - then the narrative in the race becomes a question of whether the front-runner can hang on.
However, Mr Sanders, if his recent string of victories is to be anything more than a political footnote, will need to attract black and Hispanic votes in numbers he has yet achieve.
-BBC / Reuters