Johnston, 44, served two terms in the state Senate, where he shepherded legislation to limit the size of gun magazines, expand background checks on firearm purchases and give in-state tuition to immigrants without legal status.

ADVERTISEMENT

A super PAC that backed Johnston raised more than $5 million, most of it from gun control advocates like former New York City Mayor Michael Bloomberg and LinkedIn co-founder Reid Hoffman.

“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.

“They’ve actually created new threats, like passing a tax cut that will burden our kids with debt for generations, or like trying to take health care away from the million Coloradans who need it the most,” he added.

Gardner is among the most vulnerable Republicans seeking reelection in 2020, one of only two incumbents who hold seats in states Trump lost in 2016.

A survey released this week by Keating Research, a Colorado-based pollster, found Gardner’s favorable rating at just 39 percent, while 43 percent of respondents said they viewed him unfavorably.

The same percentage of voters said they viewed Trump favorably, while 60 percent said they see the president in an unfavorable light.

"Cory is one of the most bipartisan and effective members of the Senate and he’ll continue to work hard for Coloradans across the state. We are sure this entry of yet another candidate will drag the Democratic Primary even further to the far left of the mainstream,” said Casey Contres, Gardner’s spokesman. Johnston is the fourth Democrat to say publicly he will challenge Gardner.

Former Gov. John Hickenlooper (D), who is eyeing a run for president and recently visited Iowa, is seen as the strongest potential candidate, though he has shown little interest in a run for Senate. Former state House Speaker Crisanta Duran (D) is also seen as a potential contender.

Gardner, who just ended a two-year run as chairman of the National Republican Senatorial Committee, had $1 million in his campaign account at the end of September. Candidates are required to file their end-of-year reports by the end of the day Thursday.