Republican presidential candidate Newt Gingrich lashed out at Mitt Romney on Tuesday, accusing his chief rival of a "negative smear campaign" fueled by a political action committee with close ties to the former Massachusetts governor.

"Understand, these are his people running his ads, doing his dirty work while he pretends to be above it," Gingrich told reporters after a campaign appearance at a heavy machinery plant in Ottumwa, Iowa. "I don't object to being outspent. I object to lies. I object to negative smear campaigns."

Gingrich has pledged to remain "relentlessly positive" as he campaigns for the GOP nod for the White House. He said on Tuesday that he wasn't violating that promise but simply correcting the record.

On Tuesday, Romney said in an appearance on MSNBC that super PACs have been "a disaster." But he refused to urge the group Restore Our Future to halt the attacks on Gingrich, saying that the law prohibits his campaign and such groups to coordinate.

"I'm not allowed to communicate with a super PAC in any way, shape or form," Romney said. "If we coordinate in any way whatsoever, we go to the big house."

A fired-up Gingrich read Romney's remarks to reporters and then promptly labeled them "baloney." He again urged Romney to demand that the negative spots be taken down.

Gingrich said Restore Our Future was created by Romney's former staff and funded by "his personal friends."

Gingrich's own former top aide, Rick Tyler, has joined a pro-Gingrich PAC called Winning Our Future. The former House speaker said he would expect the PAC to adhere to his positive strategy.

"If Rick Tyler runs a single negative ad, I will disown the PAC and discourage anyone from giving them a penny," he said.

"Now the governor had a very easy way to do the same thing and for him to say he couldn't find the people who gave that money and he couldn't get them to put pressure on the PAC to be reasonable is just purely dishonest," Gingrich said.

Gingrich has seen his candidacy slide in polls as a barrage of ads attacking him blanket the Iowa airwaves in advance of the state's first-in-the-nation caucuses Jan. 3. He has been trying to counter the assault while still maintaining a positive campaign.

Even so, questions over the sharp tone of the race have grabbed the spotlight, even among voters who decry heavy-handed tactics.

During Gingrich's remarks to about 100 people at the Al-Jon Manufacturing plant in eastern Iowa, one voter asked Gingrich about a political mailing he had received describing the former congressman as a globalist.

"I think these guys hire consultants who get drunk, sit around and write stupid ads," Gingrich replied. "Every one of these candidates should take responsibility for the lies they are putting up".