Illinois Sen. Mark Kirk apologized on Friday for mocking his rival Rep. Tammy Duckworth’s Thai heritage and military service during a Senate debate on Thursday evening. “Sincere apologies to an American hero, Tammy Duckworth, and gratitude for her family’s service,” the Republican incumbent tweeted. After Duckworth, who is challenging Kirk for the state’s representation in the upper chamber, described herself as a “daughter” of the American Revolution, Kirk replied with a quip many perceived as a racist at her Thai background. “I forgot your parents came all the way from Thailand to serve George Washington,” he said. Duckworth politely retorted that her father’s side of the family traces back to pre-Revolution days. Kirk, who unendorsed Donald Trump shortly before the RNC, is likely to lose his Senate seat to Duckworth if the latest poll numbers hold.