Des Moines, Iowa ? As summer turns to fall, the presidential race is heating up: Candidates are slinging elbows in debates, flaying each other in speeches and siccing media people on their party rivals. The question is which candidate takes the next step – airing the first negative advertisement of the 2008 campaign.

“We’ve seen swiping and sniping,” said media analyst Evan Tracey. “The natural progression is to take that to the airwaves and put it in an ad.”

But it’s not that straightforward. While voters may assume negative campaigning is the natural order of things, the launching of an attack ad is one of the most difficult and important tactical decisions a campaign can make.

With the balloting in the presidential race less than four months off and the holiday season looming, the timing has become acute.

In a two-person race, a negative spot runs the risk of backfiring, damaging a candidate as much or more than the intended target. The dynamic is trickier in a crowded contest, like the presidential primaries. The cycle of attack-and-response can lead to the political equivalent of murder-suicide, killing off the candidates fighting on the airwaves while boosting those watching.

Examples abound, including the 2004 race, when Democratic Iowa front-runners Howard Dean and Richard Gephardt turned the state’s airwaves into a free-fire zone and finished third and fourth, respectively, killing their White House hopes.

The calculations are especially fraught for a handful of top-tier candidates. Democratic Sen. Hillary Rodham Clinton of New York and former New York Mayor Rudolph W. Giuliani on the Republican side already are seen as combative polarizing figures; lashing out on the airwaves might simply feed that image, to their detriment.

For his part, Democratic Sen. Barack Obama of Illinois has railed against the politics of negativity and division – something the Clinton camp is quick to point out any time he is critical of the former first lady.

So given the downside, why air a negative ad in the first place?

Because elections are about choices, and to make a choice voters need to compare and contrast. A glossy, self-promotional advertisement – the type that front-running candidates typically air about themselves – may offer only part of the story. Hopefully, a negative ad will bring down a rival in the process.

The advertisements aired in the presidential race so far have been mainly of the feel-good sort – a blur of candidates shaking hands, hugging supporters, speaking resolutely. Cuddly children abound and there is lots of red, white and blue.

For all the importance of one-on-one campaigning in the early states, and for all the time spent in debates and candidate forums, TV advertising remains by far the most effective way of reaching voters.

“Only a small fraction of voters will watch any given debate or read any one newspaper article,” said Jim Jordan, a Dodd strategist. “That simply isn’t the same as reaching every caucus-goer in Iowa with a charge they will see 10, 12, 15 times in a TV advertisement.”