Cain made his remarks as Floridians were heading to the polls Tuesday for their GOP primary.
Cain told NBC's "Today" show that he endorsed Gingrich and campaigned with him Monday because he found Gingrich's tax plan similar to his own proposal, dubbed his "9-9-9" plan.
Cain said he would be "very comfortable" with Romney, too, but that both candidates carry negatives that would be attacked by President Barack Obama's re-election campaign.
Cain dropped out of the race in early December. He said he wouldn't run again, joking that his "biological clock is ticking."