Republican Nikki Haley, who until this week had run a disciplined and largely error-free presidential campaign, has been forced into an uncomfortable and unexpected new chore: doing cleanup on a blunder related to the Civil War and slavery.

The former South Carolina governor and United Nations ambassador shifted into defensive mode after she omitted slavery in comments she gave at a New Hampshire campaign event where she was asked what prompted the Civil War. Her long-winded response avoided giving the answer to a basic question most learned in grade school, teeing up a barrage of criticism from political foes.