ABC News' Shushannah Walshe and Chris Good report:

CHARLESTON, S.C. - Standing aboard the decommissioned USS Yorktown aircraft carrier this morning, an angry Rick Santorum lit into Mitt Romney for standing by a super PAC ad that accused Santorum of voting to let convicted felons vote.

The former Pennsylvania senator said he was "stunned" by Romney's appearance on Fox News' "Hannity" on Monday night, during which Romney told the cable host the ad "sounded accurate."

"He's playing dirty, dishonest politics and he's standing behind this super PAC when, in fact, he's not standing behind it anymore. He' s now saying that what they are doing, the lie they are putting out there, is a lie that he is going to stand behind," Santorum said when asked about Romney's "Hannity" appearance.

"We don't need someone who supports lies and promotes lies and stands behind those lies in order to get elected president. We need someone who is going to tell the truth to the American public, not someone who is going to deliberately lie and stand behind those lies to get elected," Santorum said, adding that Romney has a "character issue."

Santorum confronted Romney over the ad in Monday night's debate, telling the former governor that his vote, for a voting-rights bill named after the Rev. Martin Luther King, Jr., included a provision allowing convicted felons to vote again after they served their full incarceration and probation sentences. The ad depicts a prisoner in a jumpsuit.

Santorum was introduced by South Carolina state Sen. Larry Grooms, who announced he was shifting his support from Rick Perry to Santorum. Grooms said he has received seven robocalls from Romney in the past few days, including one that said Santorum was endorsing Romney. U.S. Army Lt. Gen. William Boykin (retired), a former U.S. undersecretary of defense for intelligence, also endorsed Santorum at the town hall-style meeting that followed.

Asked about his own release of a bitingly critical ad attacking Romney Monday night, less than a day after Santorum pledged he would not go negative, the former senator said he does not consider his own ad a negative one.

"I just consider that to be an ad that just tells the truth. It just lays out Gov. Romney's positions versus President Obama's position and contrasts those, well, compares them because there isn't much in the way of contrast," Santorum said. "You have Gov. Romney that again is being supported by the moderates in our party and is not going to bring the change to Washington we need and I think we need to point that out and I think we need to point that out in his ad and that's all we need."

Santorum also hit Romney on abortion and said that even if Romney wins in South Carolina Saturday, the race will not be over.

"This race will narrow down after South Carolina. It will narrow down as the primaries go on, and eventually there will be one candidate against Mitt Romney, and when that happens there will be very different elections going forward," Santorum said.

After Monday night's heated exchange over Romney's tax returns, Santorum was asked if he will release his. Santorum said he does his own and he'll be happy to release them the next time he gets home to his computer, adding, "It's not the sexiest report in the world, I assure you."