Illinois Sen. Mark Kirk has apologized for mocking his rival Rep. Tammy Duckworth's Asian heritage during a debate.
"Sincere apologies to an American hero, Tammy Duckworth, and gratitude for her family's service," GOP Sen. Kirk tweeted Friday.
During Thursday night's debate, Duckworth, who was born in Thailand and is a veteran who lost both of her legs in the Iraq War, touted her family's long history of U.S. military service. "I had forgotten that your parents came all the way from Thailand to serve George Washington,” Kirk replied. His comment sparked immediate backlash, including from Democratic presidential nominee Hillary Clinton, who tweeted at him, "It's really not that hard to grasp."
Watch Duckworth tell TIME about her history of military service here.