One of the most vulnerable Republicans in the U.S. Senate is coming under fire from members of his own party for failing to stand up to President Donald Trump.

The Lincoln Project released a blistering new attack ad this week that describes Sen. Cory Gardner (R-Colo.) as “just another Trump servant: weak, frightened, impotent.” The spot warns Gardner that he is heading for a humiliating defeat in November, with polls showing him behind potential Democratic challenger John Hickenlooper by double digits:

The ad also calls Gardner a “small man terrified of a political bully” and portrays him as protecting Trump out of fear of being on the wrong end of angry presidential tweets.

The Lincoln Project is a group of prominent anti-Trump conservatives and includes attorney George Conway ― husband of White House counselor Kellyanne Conway ― as well as GOP strategists Rick Wilson, Jennifer Horn, Steve Schmidt and John Weaver.

Horn wrote on Twitter that Gardner “betrayed his oath of office” by protecting Trump over his constituents. Conway tweeted:

If @SenCoryGardner can’t do his job, and can’t comply with his oath of office by considering the impeachment charges against Trump on their merits, then he should go.



And we at @ProjectLincoln will make sure of it, with ads like this one: pic.twitter.com/K6M2Ft3euH — George Conway (@gtconway3d) January 13, 2020

A second group of anti-Trump conservatives, Republicans for the Rule of Law, also released a new video this week calling on the Senate to allow testimony during the impeachment proceedings from former national security adviser John Bolton:

Bolton said last week that he’d testify, if subpoenaed. However, Senate Republicans have not indicated if they will allow for any witnesses at all during the upcoming proceedings.