Even as the race for the Republican presidential nomination unleashes some of the toughest attack ads of the season, the Democratic hopefuls are all sweetness and light, at least on the airwaves. After attacking Michael Huckabee on immigration last week, Mitt Romney is using a new television spot to lash into the former Arkansas governor as one who coddled convicted murderers and drug dealers.

By contrast, one of Senator Clinton’s latest ads features her mother vouching for “what a good person she is,” while a new ad for Senator Obama of Illinois features soaring music and a promise of “change we can believe in.”

Political analysts said the difference in tone between the two races is stark, but understandable in light of the strengths and weaknesses of each field and the different imperatives facing each candidate.

“It would be absolutely devastating for Hillary Clinton to go negative given the history of recent events,” a Democratic political consultant, Robert Shrum, said. “It would be absolutely inconsistent for Obama.” He also said he doesn’t think John Edwards “wants to go from being the guy leading the attacks, to trying to be nice and to benefit from that, back to the guy leading attacks again,”

“It’s largely the structure of the two races,” a professor of political science at Drake University in Iowa, Arthur Sanders, said. “On the Republican side, it seems largely to be a two-person race in Iowa. On the Democratic side, it’s a three-person race, and it’s always trickier to go negative in a three-person race. You run the risk that you hurt the person you attack and you may also hurt yourself because there’s somebody else to go to.”

Many analysts point to the 2004 Democratic presidential contest in the state, when Howard Dean and Richard Gephardt attacked each other until both lost viability.

One master strategist, President Clinton, said Friday that he expects the on-air truce among the Democratic campaigns to hold through the caucuses, even as a lot of blows are exchanged in other fora.

“In Iowa, nobody wants to go negative on television, so, really, it’s a war underneath the radar screen,” Mr. Clinton told interviewer Charlie Rose on PBS.

Aides to Mr. Obama and Mrs. Clinton are constantly on the lookout for what they view as negative attacks by the opposing camp in interviews, debates, and elsewhere.

Mr. Obama’s team is bracing for an onslaught if he pulls off an upset in Iowa. Mrs. Clinton’s campaign resents and rejects claims that it is unduly negative. Last week, the former first lady’s operation launched a Web site, AttackTimeline.com, which is devoted in its entirety to cataloging negativity by Messrs. Obama and Edwards. The charges and countercharges can be dizzying. One of the articles the site cites as an “attack” quotes Mr. Obama complaining that Mrs. Clinton’s campaign is negative.

The concern is particularly acute in Iowa, whose voters are said to abhor negative campaigning. “There probably is less tolerance of it than there is in New Jersey,” Mr. Sanders said.

Often, the most strident negative attacks are launched by political groups operating independent of individual candidates. The earliest attack ads against Mr. Huckabee this year came not from his rivals, but from an anti-tax group, the Club for Growth.

One of the groups backing Mrs. Clinton for president, Emily’s List, has taken out negative TV ads in a prior Democratic primary battle. In a congressional race in Chicago in 2002, the group tried unsuccessfully to boost its preferred candidate, Nancy Kaszak, by attacking her opponent, Rahm Emanuel, as a “Washington insider” who voted for the North American Free Trade Agreement.

Mr. Emanuel’s ad consultant in that campaign was David Axelrod, who now is overseeing Mr. Obama’s presidential bid. Mr. Axelrod did not respond to interview requests through a campaign spokeswoman.

A spokeswoman for Emily’s List, Ramona Oliver, said yesterday she didn’t expect her group to run any television ads criticizing Mr. Obama or others. “Currently, we do not have any plans to do that. I can’t speak to the future,” she said.

One obstacle to an outside group jumping into the Iowa contest at this point is that television advertising time is already scarce due to the presence of several decently well-funded contenders in both major parties.

“My understanding is there’s no time available there unless you want to talk to people at four in the morning in the middle of an infomercial,” a political strategist running independent spending for the American Federation of State County and Municipal Employees, Ricky Feller, said. He said his group plans to devote $500,000 it has already purchased in TV time in Iowa to positive ads about Mrs. Clinton’s record.

However, the union-backed political group, which has endorsed the New York senator, is mixing it up a bit with Mr. Obama. Afscme reported to the Federal Election Commission yesterday that it spent about $34,000 on a mailing that criticizes Mr. Obama.

Mr. Feller said the mailing would highlight shortcomings in the Illinois senator’s health care plan. “The real focus is who’s not covered,” the union aide said.

The chance that genteel ads will remain the norm for the Democrats into New Hampshire and beyond depends on whether any candidates with money feel “desperate” after January 3.

“I assume all bets are off after Iowa,” Mr. Shrum said.

At a news conference last week, Mrs. Clinton did not rule out ads criticizing the policy positions of her rivals. “I think contrasts are legitimate,” she said. Asked if she planned such ads regarding Mr. Obama, she said, “You’ll just have to stay tuned.”