WASHINGTON (CNN) -- Tagg Romney, the son and top campaign advisor to former Massachusetts Gov. Mitt Romney, defended Tuesday his father's changed stance on abortion rights in an interview with CNN's John Roberts.

"It's an important issue, it's a big issue, and it's something that he thought long and hard about," Tagg Romney said. "And he felt, in 2004, after studying the stem cell issue, that it was something that, the value of life had been so cheapened in this country as a result of Roe v. Wade, that he changed his mind about this issue."

"I think people, as they listen to him and hear him speak on the stump, they're very very convinced that's he's genuine and that he believes very strongly what he says," he added.

Roe v. Wade is the 1973 Supreme Court ruling that legalized abortion.

The Republican presidential candidate supported abortion rights until 2004.