Ask any political operative or observer why candidates go negative with their advertising and they’ll tell you simply, “because it works.”

But maybe there’s a limit.

On Friday, U.S. Sen. Norm Coleman, a Minnesota Republican, announced he would pull all of his negative campaign advertising from television, print and radio and disavow any interest group ads that attack his opponents.

Coleman said his decision was based on a desire to be more positive during difficult economic times. He added that he “decided I was not at all interested in returning to Washington for another six years based on the judgment of the voters that I was not as bad as the other guys. I want voters to vote for me and not against the other folks.”

There may be another, less altruistic, reason. One recent poll showed Coleman ahead in the race, but another showed him slipping behind his opponent, Al Franken, the well-known author and often crude and foul-mouthed satirist, who is running on the Democratic-Farmer-Labor ticket. Polling also showed that negative advertising might be part of the reason for a decline in Coleman’s numbers.

As usually is the case, both of the contenders were perceived as using negative advertising; it was a matter of degrees. In a poll done by the Minneapolis Star Tribune, 42 percent of respondents said ads critical of Coleman were “unfair personal attacks” while 56 percent said the same was true about ads aimed at Franken.

It is no coincidence that this race is being fueled by deep political pockets on both sides. According to the Star-Tribune the race is the most costly Senate campaign in the country. A lot of money usually means a lot of advertising and plenty of chances to try to tear down an opponent.

It will be interesting to see, first, how the Franken campaign responds to Coleman’s actions and, second, whether Coleman will be true to his positive campaign pledge all the way to Election Day. If the pledge was partially a response to the economic crisis, a stock market rebound might soften his resolve. Or if his poll numbers don’t turn around, it would be tempting to return to negative tactics.

For that matter, it also will be interesting to see exactly what qualifies as a “negative” ad in the eyes of the Coleman campaign. Partisan politics has displayed an amazing talent for rationalizing such definitions.

If Coleman follows through on this noble effort, the next question will be how voters react. Will they reward Coleman’s positive tactics or prove the experts’ contention that even though voters say they hate negative campaigning, it’s still a winning strategy?

We’d all like to think that positive, civil campaigns also can be winning campaigns. Perhaps Coleman’s decision will help set an example for other candidates to follow.