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But the decision has sparked backlash from progressives nationally and in the state. Several of the Democratic Senate candidates warned they wouldn't step aside for Hickenlooper and knocked the former governor as too moderate.

Angela Williams, a state senator who is running for the party's nomination, wrote in an op-ed for The Sentinel on Friday that Hickenlooper "needs to answer for his record and his refusal to fully embrace and support progressive solutions."

"During his short-lived presidential campaign, Governor Hickenlooper ran hard against the strong progressive energy in our party right now, and away from the big, bold, progressive solutions that our state and our country need. After spending the first half of this year campaigning against progressive ideas, he has some serious explaining to do to Colorado voters," she added.

Dan Baer, a former U.S. ambassador who is also in the race, said in a speech on Thursday that he has a "genuine personal regard" for Hickenlooper but indicated that Colorado needs "progressive leadership" in its next senator.

“We need leaders who are not conditioned by a lifetime in politics to be afraid of anything more than incremental change. That means we need different types of leaders. ... We can mend the country and win our future. We need ethical, bold, progressive leadership in Washington," he said.