Donald Trump has escalated his row with the Republican Party hierarchy, describing the presidential nomination system as "rigged" and "corrupt".
The accusations follow the Colorado primary, in which Mr Trump's rival Ted Cruz secured all 34 delegates. This came about because registered Republican voters didn't get a say -- the vote was left up to party insiders, who all sided with Mr Cruz.
This tightening of the race boosts the likelihood neither of the two frontrunners will secure the number of delegates required to win the nomination. If this happens, the party's candidate for President will be chosen at the party's convention in July, with delegates free to vote for whomever they want.
Mr Trump has now accused Republican boss Reince Priebus of playing dirty, saying he should be ashamed of himself.
"I know the rules very well, but I know that it's stacked against me by the establishment," he told CNN.
Mr Priebus responded in a tweet: "Nomination process known for a year + beyond. It's the responsibility of the campaigns to understand it. Complaints now? Give us all a break."
The Colorado branch of party itself rubbed salt into Mr Trump's wounds, sending out a tweet: "We did it. #NeverTrump."
Party officials quickly claimed their Twitter account had been hacked, and deleted the message.
The next major contest in the Republican race is New York, which Mr Trump is expected to win.