Former Colorado Gov. John Hickenlooper (D), who last week ended his 2020 presidential campaign, announced on Thursday that he will seek a seat in the U.S. Senate.

"I'm not done fighting for the people of Colorado," he said in a video attacking Washington over pre-existing conditions, prescription prices and the opening of public land to developers.

"I don't think Cory Gardner understands that the games he's playing with Donald Trump and Mitch McConnell are hurting the people of Colorado," he said, referring to Colorado's incumbent Republican senator, the president and the Senate majority leader, respectively.

"We ought to be working together to move this country forward and stop the political nonsense," he added.

Hickenlooper noted in a statement when he dropped out of the Democratic presidential primary that he was giving "serious thought" to challenging Gardner.

He was one of several Democrats running for president who had come under intense pressure to give up their White House ambitions in favor of a Senate run as the party hopes to chip away at the GOP's 53-47 majority or retake the upper chamber.

Gardner is widely considered to be one of the most vulnerable Republican senators up for reelection next year, especially given Democratic presidential nominee Hillary Clinton's victory over Trump in Colorado in 2016. The Cook Political Report, a nonpartisan election handicapper, rates the race as a "toss up."

Though Hickenlooper will certainly enter the Senate race as a top-tier contender, particularly given his high name recognition from his eight years as Colorado governor, reports have surfaced that his standing may have taken a hit in the state over his failed White House bid as he enters a primary crowd that already includes roughly a dozen candidates.

An adviser to another Democrat in the race told Politico that Hickenlooper will "have to prove his mettle."

"John Hickenlooper is desperate to redeem himself after flopping on the national stage, but we think he said it best just a few months ago: he is 'not cut out' for the Senate,'" said National Republican Senatorial Committee spokesperson Joanna Rodriguez. "This crowded Senate field has been in a race to the left and Hickenlooper's quixotic presidential bid did not do him any favors in proving he can compete in any race in 2020."

Hickenlooper repeatedly dismissed speculation earlier this year as his presidential campaign struggled to gain traction that he would consider a Senate bid should he fail to secure the Democratic nomination.

"I'm not cut out to be a senator," Hickenlooper said in February. "Senators don't build teams. Senators sit and debate in small groups, which is important, right? But I'm not sure that's my - I'm a doer. That's what gives me joy."

Updated at 7:36 a.m.