"There's a pretty compelling story that if you had a President Romney, you'd be in a different situation," Williamson told the Post. "For the first time since Jimmy Carter, we've had an American ambassador assassinated."
Obama, speaking a campaign event in Golden, Colo., Thursday, vowed that the perpetrators who killed Stevens and the other Americans in Libya would be punished.
"I want people around the world to hear me," he said. "To all those who would do us harm: No act of terror will go unpunished. I will not dim the light of the values that we proudly present to the rest of the world. No act of violence shakes the resolve of the United States of America."
ABC News' Lama Hasan, ABC News Radio and The Associated Press contributed to this report.