(Reuters) - Humbled by a stunning loss in South Carolina, Mitt Romney said on Sunday he would release this week the tax returns demanded by rivals as he bids to regain the upper hand in the volatile Republican presidential race. (Congrats to RootsAction for this victory.)

Romney, the longtime front-runner in the Republican race and one of the wealthiest presidential candidates in history, lost to a resurgent Newt Gingrich in the conservative Southern state on Saturday after stumbling badly in debates with clumsy responses to demands that he disclose his tax history.

Trying to recapture his footing as the contest heads to more populous and more moderate Florida, Romney said he would release his 2010 returns and an estimate for 2011 on Tuesday.