Speaking at a fundraising event in Des Moines, Romney twice used the word "sheikh", denoting an Arab leader, rather than "Sikh", the religion of the six people gunned down by a neo-Nazi Sunday.
"I was in Chicago earlier today. We had a moment of silence in honour of the people who lost their lives at that sheikh temple," the daily quoted Romney as saying.
"I noted that it was a tragedy for many, many reasons. Among them are the fact that people, the sheikh people are among the most peaceable and loving individuals you can imagine, as is their faith."
Rick Gorka, Romney's spokesman, defended the Republican candidate.
"He misspoke. It was the end of the day. He mispronounced similar sounding words. He was clearly referring to the tragedy in Wisconsin. You clearly heard him talk about it earlier today in Chicago," he said.