Democrat Mike Johnston, the former Colorado gubernatorial candidate and state senator, announced Thursday that he will be challenging Republican U.S. Sen. Cory Gardner in 2020, becoming the most well-known candidate yet to jump into the race.

“Donald Trump and Cory Gardner have done more than just fail to protect us from the real threats, like climate change and gun violence,” Johnston said in a video announcing his candidacy. “They’ve actually created new threats.”

Mike Johnston’s 2020 U.S. Senate campaign announcement video. Via YouTube.

Johnston ran for Colorado governor in 2018 but fell short in the primary to Gov. Jared Polis, coming in third behind former state Treasurer Cary Kennedy.

A large slate of well-known Democrats are expecting to jump into the race against Gardner, or are rumored to be considering a run. They include former state House Speaker Crisanta Duran, former House Speaker Andrew Romanoff, former U.S. Attorney John Walsh and U.S. Rep. Ed Perlmutter, among others.

Johnston, of Denver, is also a former education adviser to Barack Obama and was the principal of Mapleton Expeditionary School of the Arts.

The 44-year-old father of three ran a campaign for governor calling for education reform and tightening gun regulations — both themes he highlighted in his announcement.

“Colorado deserves a U.S. senator who will run to our hardest problems and show the leadership to pass real progressive policy,” he said. That’s why today I’m announcing my campaign for the U.S. Senate. “

Republicans were quick to attack Johnston on Thursday morning, calling him a “b-teamer” joining “what is sure to be a divisive Democrat primary.”

“The nice thing about running until you win something is you can recycle your old campaign material,” the National Republican Senatorial Committee said in an attack tweet.

Democrats already in the race to unseat Gardner include Lorena Garcia, Trish Zornio, Dustin Leitzel and Keith Pottratz.

MORE: Cory Gardner, Colorado’s Republican senator, endorses President Trump’s re-election

Gardner, who is in his first term, is seen as one of the most vulnerable Senate Republicans in the nation for 2020. The race to unseat him will be among the nation’s most watched next year and is expected to be very expensive.

Updated on Monday, March 18, 2018, at 4:30 p.m.: This story has been updated to correct the spelling of Trish Zornio’s name.﻿

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