One-time Republican front-runner Newt Gingrich told ABC News today that he has to win the South Carolina primary on Jan. 21 if he's to have a shot at being the party's nominee.

"We'll win South Carolina," Gingrich told ABC News's Jonathan Karl. "That's my must-win state."

If that's the case, Gingrich has his work cut out for him. In the latest CNN/TIME/ORC poll, Gingrich was in third place with 18 percent of the vote. Mitt Romney had 37 percent, and Rick Santorum had 19 percent.

Gingrich enjoyed a brief period atop the GOP field of candidates weeks ago, but his popularity plummeted as a so-called super PAC supported by Mitt Romney spent millions of dollars on advertising that portrayed him negatively. Gingrich ultimately came in fourth place in the Iowa caucus on Jan. 3, with 13 percent of the vote.

Now, Gingrich is pressuring Romney to call a press conference to "walk through" his history at Bain Capital, as his rivals have scrutinized his claims about the jobs he created while he was there.

"He's a guy who's made an enormous amount of money, in some cases, in ways that were very disadvantageous to the workers at companies," Gingrich said in the interview. "He's going to have to defend that."