GOP presidential hopeful Mitt Romney released a new radio ad Friday, saying he is "the only major candidate for president" to sign a pledge not to raise taxes.

Romney signed the pledge, offered by the group Americans for Tax Reform (ATR), December 31, 2006. The pledge was signed by each of the last three GOP presidential nominees -- former President George H.W. Bush, former Kansas Sen. Bob Dole, and current President George W. Bush -- during their campaigns.

Five other '08 GOP presidential hopefuls have also signed the pledge: Sen. Sam Brownback (Kansas), former Arkansas Gov. Mike Huckabee, Rep. Duncan Hunter (Calif.), Rep. Ron Paul (Texas), and Rep. Tom Tancredo (Colo.). Former Virginia Gov. Jim Gilmore, who has since renounced his candidacy, also signed.