(CNN) – The Senate race in Colorado just got uglier.
The Democratic Senatorial Campaign Committee is launching a new ad Tuesday that lifts "the curtain back on [Republican candidate] Ken Buck's tumultuous time as a federal prosecutor in the Justice Department."
The 30-second spot says Buck, who faces incumbent Sen. Michael Bennet, was rebuked for "professional misconduct," exited his job "under a cloud" and engaged in "reckless disregard."
"Buck gave inside information to lawyers for an illegal gun dealer," says the ad's narrator. "Then the dealer donated money to Buck's campaign. The official reprimand said Buck showed reckless disregard for his obligations. And Buck was ordered to attend ethics class."
Responding to the ad, Buck spokesman Owen Loftus said, "The DSCC and Michael Bennet can't run on their failed record of losing millions of jobs, pushing more Coloradans into poverty and raising the debt to $13.6 trillion."
"This type of smear and run campaign didn't work in the primary, and it's not going to work in the general election," Loftus added.
Recent polls show Buck leads Bennet by low single digits.