Samantha Power, who served on Obama's first-term national security team, made the remarks during the 2008 primary battle between Obama and Clinton and apologized the same day her remarks were made public, The Hill reported.
Power's comments were reported after Clinton beat Obama in Ohio's Democratic primary.
"We f***** up in Ohio," she said. "In Ohio, they are obsessed and Hillary is going to town on it, because she knows Ohio's the only place they can win. She is a monster, too -- that is off the record -- she is stooping to anything."
When apologizing for the remarks, Power resigned from Obama's campaign team and said the comments "do not reflect my feelings about Senator Clinton, whose leadership and public service I have long admired."
Clinton and Power worked together during Obama's first term after Clinton became Obama's secretary of state.
Power is considered the front-runner to succeed Susan Rice as the U.S. ambassador to the United Nations, The Hill said. When she was part of Obama's national security team, Power was a proponent of U.S. intervention in Libya.