(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.