Rick Santorum rebuked the pontiff and the Catholic Church saying it had been "wrong a few times on science".
Pope Francis is preparing to publish an encyclical letter, considered one of the highest forms of teaching for the pope, on the environment later this month.
Last month the Vatican leader, who has a degree in chemistry, said climate change was a sin.
"Safeguard Creation," he said, "because if we destroy Creation, Creation will destroy us! Never forget this!"
Mr Santorum, who's hoping to be the next president of the United States, has spoken of his scepticism on climate change in the past.
He said it was "not climate science, but political science".
On the Church's involvement in campaigning against climate change he said: "We probably are better off leaving science to the scientists, and focusing on what we're really good at, which is theology and morality.
"When we get involved with controversial and scientific theories, I think the Church is not as forceful and not as credible.
"I've said this to the Catholic bishops many times when they get involved in agriculture policy, or things like that, that are really outside of the scope of what the Church's main message is, that we're better off sticking to the things that are really the core teachings of the Church as opposed to getting involved in every other kind of issue that happens to be popular at the time."