As governor from 2011 until he reached his term limits in 2019, Mr. Hickenlooper employed a careful, consensus-building approach that won him praise from both sides of the aisle and helped him guide Colorado out of a recession and through a series of floods, wildfires and mass shootings in the first years of his tenure. When he left office in January, his state had one of the nation’s best economies.

He had also signed a contentious gun control package that included universal background checks, and helped Colorado become the first state in the nation to enact methane capture requirements, a measure he has said was equivalent to taking 320,000 cars off the road every year.

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But even in Colorado, fellow Democrats have expressed skepticism that his signature low-key approach will translate to national success.

“I don’t think John has at all defined why he is running,” said Rick Ridder, a political strategist and longtime friend of Mr. Hickenlooper. “There are very few people I know who wake up and want to go caucus to support a raging moderate.”

Gary Hart, the former Colorado senator and Democratic presidential candidate, predicted that Mr. Hickenlooper would appeal to primary voters because “he does not have a lot of pretensions.” But Mr. Hart noted that the candidate would have to harden his stances fast, in order to attract the most passionate party activists in the run-up to the primaries.

“They’re also going to say: ‘Fine, governor, I’m glad you can reach across the aisle, now what’s your stance on abortion?’” Mr. Hart said.

(Mr. Hickenlooper supports abortion rights, though it has not been a central issue for him.)

The former governor will kick off his campaign with a rally in Denver on Thursday, then travel to Iowa on Friday and Saturday.