One week after ending his White House bid, former Colorado Gov. John Hickenlooper launched a campaign for the Senate.

“I’ve always said Washington was a lousy place for a guy like me who wants to get things done – but this is no time to walk away from the table,” Hickenlooper said Thursday morning in a video posted on his campaign website. “I’m not done fighting for the people of Colorado.”

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Hickenlooper joins a crowded field of Democrats – he becomes the 12th candidate – bidding to unseat Republican Sen. Cory Gardner in next year’s elections.

With his presidential campaign failing to gain traction the past couple of months, Hickenlooper faced pressure from national Democrats to drop his White House bid and jump into the Senate race. Until last week, Hickenlooper had resisted such calls, repeatedly saying things like “I’m not cut out to be a senator.”

But struggling to raise money for his presidential campaign, coupled with his low poll numbers, made it all but certain by last week that Hickenlooper would fail to make the stage at next month’s third round of Democratic presidential nomination debates. In announcing his departure from the White House race, he said he would quickly make a decision on a Senate run.

The National Republican Senatorial Campaign blasted Hickenlooper, saying in a statement that “John Hickenlooper is desperate to redeem himself after flopping on the national stage."

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Hickenlooper, a geologist by training who started a successful brewery in downtown Denver and then served two terms as the city’s mayor before winning the governorship, left the governor’s office in January with an approval rating nearly 20 percentage points above water. That made him very attractive to the Senate Democratic leadership, including Senate Minority Leader Chuck Schumer, who met privately with Hickenlooper recently in New York.

Even though new polls in Colorado indicate the 67-year-old former governor would top Gardner in hypothetical general election matchups and be far ahead of the rest of the Democratic Senate contenders, his entrance into the race is not expected to clear the field. Several of the other candidates pledged last week – after Hickenlooper ended his White House bid – to stay in the race.

Democrats see Colorado as a potential pickup as they try to regain the majority in next year’s elections in a chamber the GOP controls 53-47. And they view Gardner -- a former two-term congressman who narrowly defeated incumbent Democratic Sen. Mark Udall in 2014 -- as vulnerable in 2020.

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Taking aim at Gardner and tying him to Senate Majority Leader Mitch McConnell of Kentucky, Hickenlooper said “right now we're represented by a senator who works to undo our progress by voting 99 percent of the time with Donald Trump and going along with Mitch McConnell's obstruction and partisan political games."

Even though Hickenlooper’s more moderate than some of the other more prominent Democrats running for the Senate seat, Gardner’s campaign panned Hickenlooper as “just another liberal in the clown car.”

“Whoever their party nominates will be wildly out of step with Colorado and we look forward to facing them in the general election,” Gardner’s campaign said in a statement.

Fox News' Faith Mangan contributed to this report.