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DENVER — Former state Sen. Mike Johnston said Tuesday that he is suspending his campaign in the Democratic primary for U.S. Senate.

Johnston, 44, was one of the top candidates in a crowded field vying to challenge Republican incumbent Cory Gardner next year.

But former Gov. John Hickenlooper entered the race last month after a failed presidential run, turning the race upside down.

Hickenlooper quickly gained the endorsement of several prominent Democratic state senators, members of Congress and presidential candidates as well as Democratic Senatorial Campaign Committee.

“To win this Democratic primary would now require an expensive and negative campaign,” Johnston said in a statement. “That is not who I am, and no race is worth conceding victory to a brand of broken politics that I have spent my life trying to change.”

After Hickenlooper’s announcement, several candidates already in the race, including Johnston, vowed to stay in.

Johnston ran for governor last year, only to lose the Democratic primary to Jared Polis, who waltzed to victory in the general election.

With Johnston’s departure, 11 candidates remain in the Democratic race.

Polling after Hickenlooper got into the race showed the former governor with a commanding lead over his opponents.