Republican presidential candidate Rick Santorum is interviewed on "This Week."

Rick Santorum tore into his Republican rival Newt Gingrich on Sunday, calling the former House speaker a "very high risk candidate" who could dash Republican hopes of winning back the White House, if he becomes the party's nominee.

Appearing on This Week, Santorum denounced the winner of Saturday's South Carolina GOP primary as an "erratic conservative, someone who on a lot of issues has just been wrong," adding "we have to have someone who makes Obama the issue in this race, not the Republican nominee."

Santorum told me he considers Gingrich "a brilliant guy with a ton of ideas. The issue is discipline. The issue is leadership."

The former Pennsylvania senator pointed to Gingrich's tenure as House speaker, telling me "If you look at his leadership… there was a conservative coup within three years of him becoming the Speaker and eventually he was forced out because of, well, you know, issues of being able to focus, execute, discipline."

Santorum also noted that Gingrich's problems didn't end with his congressional career, citing the candidate's one-time repudiation of Rep. Paul Ryan's Medicare revamp as "right wing social engineering" and last summer's mass exodus of the Gingrich campaign staff.

"This is not, you know, the old Newt," he said. "This is the last six months."

For Santorum, it's been a week of highs and lows. He was declared the official winner of the Iowa caucuses, but then went on to a disappointing third place finish in South Carolina.

On Sunday, Santorum told me he would reject any calls from conservatives that he drop out of the race.

"I've beaten Mitt Romney," he said. "Newt Gingrich has beaten Mitt Romney. The idea that conservatives have to coalesce in order to beat Mitt Romney, well, that's just not true anymore.  Conservatives actually can have a choice."

Watch my full interview below.