Former Colorado state Sen. Mike Johnston (D) said Thursday he will challenge Sen. Cory GardnerCory Scott GardnerSenate advances public lands bill in late-night vote OVERNIGHT ENERGY: Interior faces legal scrutiny for keeping controversial acting leaders in office | White House faces suit on order lifting endangered species protections | Lawmakers seek investigation of Park Police after clearing of protesters The Hill’s Campaign Report: Republicans go on attack over calls to ‘defund the police’ MORE (R) in 2020, the highest-profile contender to enter the race against Gardner so far. 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. ADVERTISEMENTHe ran for governor in 2018, finishing third in the Democratic primary behind Rep. Jared PolisJared Schutz PolisState leaders urge protesters to get tested for coronavirus amid fears of new outbreaks The Hill’s Morning Report – Protesters’ defiance met with calls to listen Overnight Health Care: White House shifts focus from coronavirus | House Democrats seek information on coronavirus vaccine contracts | Governors detail frustrations with Trump over COVID-19 supplies MORE (D), who won the general election. 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. In his announcement video Thursday, Johnston sought to tie Gardner closely to President TrumpDonald John TrumpSenate advances public lands bill in late-night vote Warren, Democrats urge Trump to back down from veto threat over changing Confederate-named bases Esper orders ‘After Action Review’ of National Guard’s role in protests MORE, who lost Colorado’s electoral votes by 5 percentage points in 2016. “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. Gardner won election in the Republican wave of 2014, beating out Sen. Mark UdallMark Emery UdallThe 10 Senate seats most likely to flip Democratic presidential race comes into sharp focus Democrats will win back the Senate majority in 2020, all thanks to President Trump MORE (D) in one of the most significant upsets of the year. But he faces headwinds in a state that has trended toward Democrats in recent years. Republicans have not won a race for governor since 2002. In 2018, Democrats won all five state constitutional offices, and Democrat Jason CrowJason CrowGun control group rolls out House endorsements Bipartisan House bill seeks to improve pandemic preparedness Human Rights Campaign rolls out congressional endorsements on Equality Act anniversary MORE beat Rep. Mike CoffmanMichael (Mike) Howard CoffmanBottom Line Koch political arm endorses Colorado Sen. Gardner 20 years after Columbine, Dems bullish on gun reform MORE (R) in a suburban swing district around Denver. 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.