DENVER (CBS4) – Mike Johnston, a Democrat who ran for governor in Colorado last year and lost in the primary election, wants to challenge Republican Sen. Cory Gardner for the U.S. Senate. Johnston lost out to Gov. Jared Polis in the statewide primary last June.

Mike Johnston (credit: CBS)

READ MORE: Kit Carson Peak Climber Madeline Baharlou-Quivey's Body Retrieved By Rescue Crews, Helicopter

Johnston made his announcement Thursday morning. So far, he’s the most high profile Democrat who will be running to be the Democratic nominee next year.

He wrote on Twitter: “I’m Mike Johnston. I’m a former teacher, principal, Obama advisor, and State Senator. And, today, I’m announcing my Democratic campaign to defeat Republican Senator Cory Gardner. Help Democrats flip this critical seat.” He also made the announcement in Spanish in a corresponding tweet.

READ MORE: Latino Community Across Colorado Prepares To Celebrate Día De Los Muertos

The National Republican Committee wrote in their own tweet shortly after Johnston’s announcement: “Another b-teamer throws their hat in the ring for 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.”

Sen. Cory Gardner (credit: CBS)

MORE NEWS: 'Live Like Her': Sally Strelecki Taken Off Life Support After Bullet From Neighboring Apartment Kills Her

In an article posted Wednesday, the Associated Press referred to Gardner, Colorado’s junior senator, as being “widely seen as the most vulnerable GOP incumbent in 2020.” The article revealed that Gardner will back President Donald Trump’s re-election bid in 2020.