Historian and author Nancy L. Cohen explained to Reason TV on Monday the powerful influence of "sexual fundamentalists" on the Republican Party.

"About forty years ago, everything changed in America," she said. "Birth control actually used to be illegal, gay sex was illegal, and then you had this sexual revolution. What happened is a small group of Americans -- mostly religious fundamentalists -- freaked out about these changes in sexual relations and personal freedom and they became politically-active exactly and deliberately to roll back these changes."

Cohen said sexual fundamentalists comprised about 50 percent of the Republican electorate and had become the "gatekeepers" of the 2012 Republican primaries.

"That is why we're seeing someone like Santorum, who is really a fringe candidate and it's also why Mitt Romney has been forced to talk about birth control and abortion much more than he would want to," she said.

Watch video, courtesy of Reason TV, below: