Mike Johnston ended his Senate campaign in Colorado on Tuesday, becoming the first Democrat to leave the race after former Gov. John Hickenlooper launched his campaign last month.

Johnston, a former state senator who also ran for governor last year, was the first well-known Democrat to enter the race against GOP Sen. Cory Gardner in January and led the field in fundraising through the first two quarters of this year. His campaign had more than $2.6 million on hand as of June 30, more than double any of his opponents. But he said Hickenlooper's entrance as a candidate would have altered his campaign in a negative way.

"The more we looked at it, the more I realized that to win the Democratic primary now would require a really expensive and I think a really negative campaign. And that's not who I am, and that's not what I believe," Johnston said in an interview. "I think that no race is worth conceding victory to a broken brand of politics I spent my life trying to undo."

He added that a grueling and expensive primary "would have risked the chance of weakening the eventual nominee so that we could have lost to Gardner and lost the seat, and that's the opposite of what everybody that came to our campaign came to accomplish." Colorado is a must-win state for Democrats to have any path back to the Senate majority.

Johnston said he had no immediate plans to endorse one of the other Democrats in the primary, though he did not rule out making an endorsement down the road. He also said he had not spoken to Minority Leader Chuck Schumer or anyone at the Democratic Senatorial Campaign Committee since Hickenlooper announced his bid. "There's been no pressure from them or anyone else for me to get out," he said.

He added that he was not encouraging other candidates in the race to follow his lead and drop their bids, but that he saw avoiding an expensive primary as the most productive path forward. He said the Supreme Court, action on climate change and "democracy reform" were at stake in the race.

"Whatever the existential question is that you're most passionate about, every one of those questions rely on a win in the U.S. Senate race in Colorado. And that is much bigger than us," Johnston said. "I think when you're part of a team you've got to do what's best for all of us, not what's best for one of us. And this seemed like the clear right choice for me."

Hickenlooper praised Johnston in a tweet, calling him a "friend, a tremendous public servant and a great Coloradan. He’s always put the good of the state and indeed country first."

Johnston's departure likely puts more pressure on the remaining Democrats — nearly a dozen are in the race — to consider dropping their bids as well. Most of the candidates said after Hickenlooper launched his campaign that they planned to continue running, but many Democrats in Colorado and Washington expected the field to shrink.

Hickenlooper's entrance in the race frustrated liberals in the state, but polling has shown him to be a prohibitive frontrunner in the Democratic primary. The DSCC endorsed his bid last month, a move which elicited backlash from other candidates in the race.