The comments were swiftly condemned by Republican National Committee Chairman Reince Priebus, who insisted this was the same nominating process the party has "been using for many years."
Headlined "Let me ask America a question,"
Trump wrote in a Wall Street Journal op-ed Thursday night, "How has the 'system' been working out for you and your family?"
Trump condemned Colorado's delegate selection process as undemocratic. "One million Republicans in Colorado were sidelined," he wrote, challenging Americans to judge the current "system."
"I, for one, am not interested in defending a system that for decades has served the interest of political parties at the expense of the people," the Republican front-runner declared. "No one forced anyone to cancel the vote in Colorado. Political insiders made a choice to cancel it. And it was the wrong choice."
The Republican Party in Colorado last year decided to forgo holding an open presidential primary or caucus contest for the 2016 cycle due to cost concerns, choosing instead to have party members select delegates at the state convention.
Trump also sought to link primary rival Texas Sen. Ted Cruz to those insiders seeking to deny him the nomination. "The great irony of this campaign is that the 'Washington cartel' that Mr. Cruz rails against is the very group he is relying upon in his voter-nullification scheme," he wrote.
"My campaign strategy is to win with the voters," Trump wrote. "Ted Cruz's campaign strategy is to win despite them."
But Priebus defended the nominating process Friday in an interview with CNN's Wolf Blitzer, saying that if Trump doesn't like the rules, he can change them.
"This is a very normal system that we've been using for many years," Priebus said on "Wolf." "And by the way, if anyone wants to reform the system, they can do so on the Rules Committee, because every four years, the Rules Committee gets together and they write the rules for the nomination of our party. So that's who writes the rules. It's not the RNC that writes the rules to determine the system."
Also on Friday, Sean Spicer, the RNC communications director, reiterated that "the rules surrounding the delegate selection have been clearly laid out" for months, and noted that delegates can be awarded "through a primary, caucus, or convention" -- the last being the particular target of Trump's frustration.
But despite Trump's charge in his piece that "what we are seeing now is not a proper use of the rules, but a flagrant abuse of the rules," the RNC adamantly resisted those claims. "The RNC is transparent about the rules and works with campaigns on a consistent basis to address any questions surrounding the process," Spicer wrote in Friday's memo.
Seeming to respond indirectly to Trump, he also wrote, "it ultimately falls on the campaigns to be up to speed on these delegate rules."
"Each process is easy to understand for those willing to learn it."
Later Friday afternoon, Steve House, the chairman of the Colorado Republican Party, said in a statement that "the notion that any secret group of politicians colluded behind closed doors against one presidential candidate last August by eliminating the straw poll is completely false."
"Considering turnout at caucus was about the same in 2016 compared to 2012, it's clear the straw poll is not what 'sidelines' Colorado Republicans -- it is state law which does not provide Coloradans with a presidential primary," said House, who added that the state party "stands with all those today calling on the state legislature to open up the presidential voting process by instituting a true primary."
Speaking in Plattsburgh, New York, Friday afternoon, Trump continued his criticism of the delegate process in Colorado.
"They changed the system," Trump said. "And they went to a deal where the bosses picked the delegates. And the people never got to vote. And the people didn't know in Colorado that their vote was being taken away from them and let me tell you, you have some angry people in Colorado right now. And you want to know the truth? It's a beautiful thing to watch because they're 100% right."
Trump kept up his attacks later in the evening, telling an audience in Hartford, Connecticut, "If I don't make it, you're going to have millions of people who don't vote for a Republican."
Trump then seemed to allude to a comment he made last month
in which he warned of riots if the GOP establishment has a contested convention.
"You're going to have millions of people that aren't going to vote -- and hopefully that's all. Hopefully that's all," Trump said Friday.