(Bloomberg News) As Iowa Republicans take in the final appeals before tomorrow's caucus voting, one major lesson the candidates will take away: Going negative works.

An estimated $5.8 million was spent on television advertising in Iowa through Dec. 30, with $3.7 million financing negative ads, according to most recent data available from New York-based Kantar Media's CMAG, a company that tracks advertising.

Most of those negative ads were directed against one candidate: former House Speaker Newt Gingrich, who led national and Iowa opinion polls from mid-November to mid-December.

Since Dec. 1, 45 percent of all ads airing in Iowa criticized Gingrich for shifting policy positions and advocacy for Freddie Mac, a government-backed mortgage-finance company caught up in the housing crisis, and other groups after resigning the House in 1999.

The commercials were primarily financed by Texas Republican Congressman Ron Paul, Texas Governor Rick Perry and an outside committee that backs former Massachusetts Governor Mitt Romney, who used his own campaign cash to run only commercials promoting himself.

"The big story is what a huge proportion of the advertising was spent against Gingrich," said Ken Goldstein, president of CMAG. "In a multicandidate race, to have such a huge proportion of the advertising focus on a negative way on one person, with him unable to respond, is really unprecedented."

Scant Positives Ads

According to CMAG's data, 6 percent of Iowa television advertising since Dec. 1 supported Gingrich's candidacy.

The anti-Gingrich ads began as he became the fifth candidate to rise in opinion polls as the alternative to Romney.

"That momentum had to be quickly broken, and everybody jumped on the bandwagon do to it," said Eddie Mahe, a Republican consultant.

Since the start of the campaign, an independent committee supporting Romney's campaign, Restore Our Future, has aired 1,746 commercials -- all of them negative, and most aimed at Gingrich. That committee is run by Romney's political allies, although it can't legally coordinate with his campaign. While Restore Our Future took over the anti-Gingrich messaging, Romney used his campaign cash to air 2,049 positive commercials about his candidacy.

Gingrich also was the subject of a hard-hitting ad by Paul, which was titled "serial hypocrisy." In all, Paul has run 2,572 commercials, of which 1,310 were attack ads.

Freddie Mac Criticism

Criticism of Gingrich focused extensively on his appearance in a climate change advertisement with House Minority Leader Nancy Pelosi, a California Democrat, and his $1.6 million in earnings as an advocate for Freddie Mac, seized by the government in 2008 after its stake in subprime loans pushed it to the brink of collapse.

By contrast, Gingrich had financed 1,215 commercials through Dec. 30, all of which were positive, according to the CMAG data.

The onslaught of negative attacks has coincided with a decline in support for Gingrich in polling. A Dec. 31 Des Moines Register Iowa Poll of likely caucus attendees put Gingrich in fifth place with 12 percent support; an Iowa Poll released on Dec. 3 had him in first place with 25 percent.

Gingrich, who has called the advertising targeted at him "dishonest," criticized his rivals at a town hall meeting Dec. 30 in Des Moines, Iowa.

Shameful Ads

"I would be ashamed to run some of the ads they are running and I will not participate in that kind of process," Gingrich said. "I can't be a witness to America's future while smearing my opponents and I would simply ask every Iowan before Tuesday night to ask yourself, 'Do you really want to reward the consultants, the lies, the negativeness, or do you want to say to the country, 'A brand new day has arrived.'"

Brittany Gross, a spokeswoman for the pro-Romney Restore Our Future committee, declined to comment. Andrea Saul, a spokeswoman for Romney, did not respond to a request for comment.

Romney defended the ads financed by Restore Our Future in a Dec. 21 interview on Fox News and said they helped bring down Gingrich's ratings.

Romney Defends Ads

They "pointed out some of the aspects of his record that people weren't aware of and that's brought the numbers down," Romney said on Fox. "By the way, there's nothing that any of these ads by any of the candidates are showing about Speaker Gingrich that President Obama wouldn't put out with his billion- dollar money that he's going to have down the road. So it's probably a good time for people to see these things to make up their mind."

Other candidates received less negative attention, CMAG data show. Twenty percent of the ads attacked Romney. About eight percent of the ads mentioning Perry were negative.

Mahe said the negativity in Iowa is only a preview of what the Republican nominee, and President Barack Obama, will face later.

"This campaign will be nothing but negative ads on all sides," Mahe said. "We know Obama will, because that's the only story he's got. Obviously, as we've seen, there's a lot things folks have to say about Obama that could be characterized as negative."