Democrat announces challenge to GOP US Sen. Cory Gardner

Mike Johnston. Photo courtesy: Colorado Legislature
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DENVER (AP) - Republican Sen. Cory Gardner has his first major Democratic challenger.

Colorado State Senator Mike Johnston on Thursday announced he was running against Gardner. Johnston is a former Obama campaign adviser who ran unsuccessfully for the Democratic gubernatorial nomination in Colorado last year.

Gardner is viewed as the most vulnerable Republican senator in the country. The state supported Hillary Clinton in 2016. Last year Democrats won every statewide race in Colorado. Gardner on Wednesday announced he'd endorse Trump's re-election.

Other major Democrats who may run against Gardner include two former state House Speakers, Crisanta Duran and Andrew Romanoff.