MYRTLE BEACH, S.C. - Though he pledged just hours ago to keep his campaign "above board," and not go negative, Rick Santorum unveiled a bitingly critical new campaign ad that portrays rival Mitt Romney and President Barack Obama as political clones.

"Obama supported the Wall Street bailouts… So did Romney," the ad says over photos of the president and the former Massachusetts governor. "Obama gave us radical 'Obamacare,' that was based on 'Romneycare.' Obama's a liberal on social issues. Romney once bragged he's even more liberal than Ted Kennedy on social issues. Why would we ever vote for someone who is just like Obama - when we can unite around Rick Santorum and beat Obama?"

With six days to go before voters go to the polls in South Carolina, the campaign has unleashed its first negative ad. Earlier televisions spots focused on his conservative credentials and close family life.

This morning at campaign stop in Columbia, he addressed the negative ads a pro-Romney super PAC is running against him in this state, and was asked specifically whether he would respond in kind.

"What you've seen me on the campaign is what I'm going to do on our advertising, which is to talk about my record, if I'm going to juxtapose it versus how I've voted or what I believe that I've done here versus others," Santorum said at the Lizard's Thicket restaurant. "I'll do that, but I'm not going to go out there and misrepresent someone's record for political purposes. I don't do that, I don't believe in it.

"The people of South Carolina deserve an honorable campaign, someone who goes out and tells the truth, someone who talks about their record and juxtaposes it with someone else on a factual basis, and we are going to do that, as we have in this campaign, and we're going to keep it above board, we're going to keep it about issues, not about things that clearly are intended to mislead the people of South Carolina," he said.

An ad running in the Palmetto State sponsored by the super PAC Restore Our Future hits Santorum on earmarks and accuses him of "voting to allow convicted felons vote."