A group run by anti-Trump conservatives has released an advertisement targeting one of the most vulnerable Republican senators, Colorado Sen. Cory Gardner.

Gardner, 45, has one of the toughest reelection campaigns in the nation as the centrist Republican attempts to maintain his seat with President Trump on the ballot next to him. Trump lost Colorado in 2016 by 4.9%, and the Cook Political Report listed Gardner’s seat as a toss-up .

The Lincoln Project, a group led by George Conway, the husband of Trump’s counselor Kellyanne Conway, Steve Schmidt, John Weaver, and Rick Wilson, targeted Gardner for siding with Trump on several issues. The advertisement opens with the question, “Why are you losing?” and proceeds to blame Gardner’s tight reelection race on his relationship with Trump.

“Why do the polls show you headed for humiliating defeat in November? Because Colorado trusted you to work for us, not Donald Trump,” the video says. “Tough people built our great state, and they’ve always fought for what’s right. The only thing you’ll fight for is Trump. You’re just another Trump servant: weak, frightened, impotent, a small man terrified of a political bully.”

Cory Gardner, do your job. pic.twitter.com/98QpeewTD8 — The Lincoln Project (@ProjectLincoln) January 13, 2020

The message focused on Gardner’s role in the upcoming Senate impeachment trial and claimed that the senator would be covering up for Trump if he does not vote to allow further witnesses to testify.

Gardner will likely face former Colorado Gov. John Hickenlooper, who made a brief presidential bid last year. Hickenlooper, 67, faces a primary race in 2020, and several of his fellow Democratic candidates have given him an icy welcome to the Senate race after he dropped his presidential campaign.