John Hickenlooper, the former Colorado governor who ended his presidential campaign last week, announced on Thursday that he would run for a United States Senate seat in his home state, instantly making him one of the Democrats’ best hopes in their quest to retake the chamber next year.

Mr. Hickenlooper was resistant to the idea of running for the Senate while he was still in the presidential race, telling reporters that he was “not cut out to be a senator.” But discussions about a bid grew serious in recent weeks as his campaign faded, senior aides abandoned him and he failed to secure a spot in the next Democratic debate.

“I’ve always said Washington was a lousy place for a guy like me who wants to get things done,” Mr. Hickenlooper said in a video posted on his campaign website Thursday morning. “But this is no time to walk away from the table.”

Mr. Hickenlooper, who is also a former Denver mayor, brewpub owner and geologist, will seek to challenge Senator Cory Gardner in a contest that Democrats view as all but necessary to win if they have any hope of flipping the Senate in 2020.