(CNN) – Newt Gingrich said Wednesday a relentless attack from his rivals ultimately eroded his will to stay positive in the race for the GOP nomination.

Speaking with CNN's Piers Morgan, the former House speaker said Mitt Romney's refusal to disavow attack ads produced by a super PAC sponsoring his candidacy left him with no choice but to go negative.

- Follow the Ticker on Twitter: @PoliticalTicker

"I concede that every effort I made to stay positive and every effort I made to talk Romney out of doing this failed," Gingrich said. "But you can't - you know you can't unilaterally disarm unless you want to get out of the race. And since this is the objective reality we have no choice."

A super PAC supporting Gingrich's own candidacy recently purchased more than $3 million of ad space in South Carolina, and is preparing to unleash a scathing attack on Romney's time as CEO of the private investment firm Bain Capital.