The Republican presidential candidates are shying away from direct attacks on each other as they head into a key Iowa straw poll, training their fire instead on their Democratic counterparts over foreign policy and taxes.

In a debate broadcast on ABC’s “This Week” yesterday from Des Moines, Iowa, Senator Obama of Illinois emerged as the favored punching bag of the Republican hopefuls, who criticized both his willingness to meet personally with rogue dictators and his pledge last week to order a unilateral military strike to target terrorists in Pakistan if its president, Pervez Musharraf, refused to take action.

“In one week he went from saying he’s going to sit down, you know, for tea with our enemies, but then he’s going to bomb our allies,” Mitt Romney said, before drawing laughter with what appeared to be a preplanned shot at Mr. Obama: “I mean, he’s gone from Jane Fonda to Dr. Strangelove in one week.”

The Republican contenders sniped over social issues but presented a largely united front on broader issues of fighting terrorism and opposing an expanded government role in health care. With the exception of one candidate, Rep. Ron Paul of Texas, they all voiced support for the Iraq war, and several expressed optimism at recent reports indicating the American military has made strides in securing parts of the country. Mr. Paul, who is running on an anti-intervention, libertarian message, called for the immediate withdrawal of troops and said America is losing the war.

Mr. Romney, a former Massachusetts governor who is leading in the Iowa polls, found himself at the center of the debate’s most heated exchange when he tried to fend off an attack from Senator Brownback of Kansas over his record on abortion. The debate moderator, George Stephanopoulos, confronted Mr. Brownback at the outset about telephone calls his campaign is making to voters in Iowa that highlight Mr. Romney’s previous backing for abortion rights and contributions his wife has made to a prominent abortion rights organization Planned Parenthood.

Mr. Brownback, who has run as a staunch social conservative, stood firmly behind the ad. “There’s one word that describes that ad, and it’s ‘truthful.’ It’s a truthful ad.” he said.

“Virtually nothing in that ad is truthful,” Mr. Romney shot back. “The single word I’d use is ‘desperate’ or perhaps ‘negative.'”

He explained, as he has done repeatedly throughout the campaign, that while he previously supported abortion rights, he changed his mind in 2004 when as Massachusetts governor he dealt with the issue of embryo cloning.

“I get tired of people that are holier than thou because they’ve been pro-life longer than I have,” Mr. Romney later added.

The dispute comes as several of the Republican candidates are gearing up for Saturday’s Ames straw poll, a traditional test of campaign strength in Iowa that this year may hold more significance for lesser-known contenders such as Mr. Brownback. Mayor Giuliani and Senator McCain of Arizona are not actively competing in the event, leaving Mr. Romney as the only top candidate to participate and the heavy favorite. The other candidates, however, are jostling aggressively for a strong showing that could propel one or more of them into serious contention for the state’s first-in-the-nation caucus in January.

With Mr. Giuliani absent from the straw poll, Mr. Romney appeared less than eager to criticize him directly, refusing a chance yesterday to repeat comments he made in March about Mr. Giuliani’s support for abortion rights, gay rights, and gun control.

The closest Mr. Romney came to taking a direct shot at Mr. Giuliani was on health care, where the former mayor recently unveiled a plan aimed at expanding access and reducing costs through tax deductions and health savings accounts. “We have to have our citizens insured, and we’re not going to do that by tax exemptions, because the people that don’t have insurance aren’t paying taxes,” Mr. Romney said.

The candidates stuck close to the conservative principles of lower taxes and smaller government, even at the risk of offending a key Iowa lawmaker. They all said they opposed a push by a fellow Republican, Senator Grassley of Iowa, to fund an expansion of the federal Children’s Health Insurance Program through an increase in the cigarette tax.

At least for a day, Mr. Obama replaced Senator Clinton as the no. 1 Democratic target among Republicans, as Mr. Romney, Mr. Giuliani, and a California congressman, Rep. Duncan Hunter, all assailed his foreign policy agenda. An Obama spokesman responded yesterday that the Republican debate “proves why Americans want to turn the page on the seven years of Bush-Cheney foreign policy.”

Mr. Giuliani also criticized Democrats for what he said was a “knee-jerk liberal Democratic reaction” to raise taxes, along with health care proposals that would lead to “socialized medicine.”

Several of the nine hopefuls tried to deflect two potentially troublesome questions, one inviting criticism of Vice President Cheney’s influential role in the Bush administration and another asking the candidates to name “the defining mistake” of their lives.

“I would be very careful that everyone understood there was only one president,” Mr. McCain said, answering the question about what authority he would delegate to the vice president.

And asked about his “defining mistake,” Mr. Giuliani, whose personal travails have been well-documented, went for humor. “To have a description of my mistakes in 30 seconds,” he replied to laughter. Then, turning to Mr. Stephanopoulos, he said. “Your father is a priest. I’m going to explain it to your father, not to you, okay?”

For the fourth consecutive Republican debate, the missing man was a former Tennessee senator, Fred Thompson, who has been “testing the waters” for a presidential bid and is likely to declare his candidacy shortly after Labor Day. A less conspicuous absence was that of a former Virginia governor, James Gilmore, who quietly dropped out of the race last month.