A labor union boss has resigned after running a controversial political attack ad that targeted a doctor whose family was killed in a horrific 2007 Connecticut home invasion.

Fox 61 reports that Paul Filson's resignation Thursday as executive director of the 55,000-member SEIU Connecticut State Council came in the face of widespread criticism of the ad in which Dr. William Petit and GOP presidential candidate Donald Trump are accused of an “attack on women and families.”

Filson ran a political action committee that produced the online ad with money from the SEIU and other Connecticut labor unions, the station reported.

Petit was the only survivor in the 2007 Cheshire, Conn., home invasion in which his wife and two daughters were killed. The convicted killers were sentenced to death but are now serving life sentences in prison after the state abolished the death penalty.

The doctor is a Republican candidate for the Connecticut state legislature and is locked in a tight race against 11-term incumbent Democrat Rep. Betty Boukus.

The SEIU in a statement announcing Filson’s resignation apologized to Petit and said the ad should have never run “in his district under any circumstances," the station reported.

Petit denounced the ad at a press conference Wednesday. “I’m appalled that anyone would stoop this low,” he said, according to the station.

Fox 61 reported that Boukus joined him at the press conference. She also denounced the ad and said she had nothing to do with it.

The Hartford Courant reported that at first Filson rejected criticism the ad was underhanded.

He later expressed remorse, according to the paper.

“We can only imagine the pain that Petit has gone through, and would never target his loss for our political agenda,” he said, according to the Courant.

Click for more from Fox 61.