Republican presidential candidate Donald Trump has described as “rigged” the way the US state of Colorado picks its presidential nominee.
His rival Ted Cruz swept the board in that state, where delegates are selected by senior party activists.
Mr Trump is still well ahead in the Republican race but if he does not get 1,237 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 can 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.
Senior Republicans have voiced concern about Mr Trump’s chances of winning in November’s general election, and oppose his hardline views on immigration.