While reiterating his determination to run a positive campaign, Newt Gingrich took a veiled swipe at Mitt Romney over an attack ad now airing in Iowa, where voting in the Republican presidential contest is less than three weeks away.

Gingrich, speaking to reporters in Iowa City, said that maintaining his lead in the polls ahead of the Jan. 3 caucuses is "going to be a challenge because you have everybody firing away simultaneously in a relatively small market."

Gingrich said that because he has less campaign money than some of his competitors, "we're not going to have as many ads as they have, and that's a fact."  

Then he drew a bead on an attack ad being run by a pro-Romney super PAC.

To counter the negative assault, the former House speaker said, it is "going to require two weeks of my going around [Iowa], telling the truth, letting people look at the negative ad, look at the truth and decide:  Do they really want to give their vote to somebody who is not telling the truth?"

Gingrich declined to say which candidate he was referring to.  But the substance of his remarks -- which focused on a claim in the Romney PAC ad questioning Gingrich's conservative credentials -- left little doubt that he was referring to the former Massachusetts governor, or in this case, his supporters.

After rattling off a series of accomplishments, including his role in passing balanced budgets through Congress and overhauling welfare, Gingrich said that "for somebody to suggest that that's not a conservative voting record, I mean, at some point it becomes a joke. ... It tells you more about the person who would run the ad than it tells you about Newt Gingrich."

Gingrich plans to barnstorm Iowa between Dec. 27 and Jan. 3, aboard a customized bus similar to 2008 Republican candidate John McCain's Straight Talk Express.

"We'll have our very own bus," he said. "It'll be a lot of fun. I'm looking forward to it."

He shrugged off a finding in the latest NBC News/Wall Street Journal poll that showed him trailing President Obama in a test match-up, while Romney fared better against the incumbent president.

Gingrich said he thought he was doing about as well, at this point in the contest, as candidate Ronald Reagan did against President Jimmy Carter in late 1979.

"Reagan doesn't catch up with Carter until September of 1980," Gingrich said.