An embattled first-term United States senator, Mark S. Kirk of Illinois, faced sharp criticism after seeming to mock his Democratic challenger’s ethnicity and her family’s history of military service at a debate on Thursday night.

Hillary Clinton and other Democrats on Friday condemned Mr. Kirk, a Republican, and urged him to apologize to his opponent, Representative Tammy Duckworth, a veteran who lost her legs in the Iraq war, after his retort at the debate in Springfield, Ill.

A little after noon on Friday, he offered his “sincere apologies” to Ms. Duckworth on Twitter.

The moment came after Ms. Duckworth spoke about the human costs of war. “My family has served this nation in uniform, going back to the Revolution. I’m a daughter of the American Revolution. I’ve bled for this nation,” she said.



“But I still want to be there in the Senate when the drums of war sound,” she added. “Families like mine are the ones that bleed first. But let’s make sure the American people understand what we are engaging in, and let’s hold our allies accountable, because we can’t do it all.”