"I don't care if she's as good looking as Taylor Swift or as nice as Mr. Rogers, but if she votes like Michele Bachmann, she's wrong for the state of Iowa," the retiring Iowa Democrat can be heard saying at the Story County Democrats' fall barbecue, in footage published Sunday night.
Harkin has frequently campaigned for Ernst's opponent, Rep. Bruce Braley, D-Iowa.
Ernst, who has led by tiny margins in many recent polls, slammed Harkin's comments as sexist.
"I was very offended that Sen. Harkin would say that," she told Fox News Monday morning. "I think it's unfortunate that he and many of their party believe you can't be a real woman if you're conservative and you're female. I believe if my name had been John Ernst attached to my resume, Sen. Harkin would not have said those things."
Ernst said Harkin's comments were just the latest in Democrats' "phony war on women."
"I am a woman and second I have been to war. I am a combat veteran, this is not a war on women and anytime Democrats use the word war, they need to do it to honor our servicemen and women," she said.
Later Monday, Harkin backtracked.
"I shouldn't have said those things, I know that. I regret anytime someone feels offended by what I have said," Harkin said. "But I am only human and I can make mistakes sometimes in how I say something. In fact, I have complimented her on running a very good campaign."