A recent poll showed Mr. Hickenlooper with more than a 50-point lead over the current leading Democrats in the race for the party’s nomination for the Senate seat; another poll showed him ahead of Mr. Gardner by 13 percentage points in a head-to-head matchup. Those surveys were conducted by Democratic polling firms.

After Mr. Hickenlooper’s announcement, the advocacy group that commissioned one of the polls urged the former governor to challenge Mr. Gardner.

“As mayor of Denver and governor, John Hickenlooper did what was right for Colorado, and what is right for Colorado now is for him to defeat Cory Gardner,” said Josh Morrow, the executive director of the group, 314 Action Fund. “In the Senate, Gardner has pushed the far-right Trump agenda harder than Trump himself.”

A spokesman for Mr. Gardner’s Senate campaign did not respond to a request for comment on Thursday.

Mr. Hickenlooper is one of a handful of presidential candidates, including former Representative Beto O’Rourke of Texas and Gov. Steve Bullock of Montana, who have been encouraged by some Democrats to drop their bids for the White House and run for the Senate.

At first, he was largely resistant to the idea. But this month, as his presidential campaign lurched along, Mr. Hickenlooper’s communications director told CNN that the former governor had not “closed the door to anything.” Senator Chuck Schumer, the minority leader, has spent months trying to recruit Mr. Hickenlooper. And during a recent trip to Iowa, Mr. Hickenlooper hopped into the car of Michael Bennet, Colorado’s Democratic senator and another presidential candidate, to discuss his impending decision.

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In kicking off his presidential campaign, Mr. Hickenlooper pitched himself as a unifier who could help mend what he called a “crisis of division.” But in a partisan era in which many Democrats are seething with anger toward President Trump, messages about compromise and compassion from Mr. Hickenlooper and some of his rivals have largely fallen on deaf ears.

On policy, he sought to carve out space for himself as a moderate option for voters during an election cycle that has seen progressive ideas flourish. A successful entrepreneur who helped open a chain of Midwestern pubs and restaurants, Mr. Hickenlooper staunchly defended capitalism and rejected socialism — even when it earned him disdain.