"It would be OK to repeal it. It would be OK also if a strict constructionist viewed it as precedent," said the former New York city mayor, who has a record of supporting abortion rights.
In a party that draws strength from anti-abortion voters, Giuliani's nine GOP rivals agreed that it would be a great day if the court overturns the landmark ruling.
"Glorious day of human liberty and freedom," enthused Sen. Sam Brownback of Kansas.
Former Massachusetts Gov. Mitt Romney acknowledged he had changed his mind on the subject when he began to delve into the issue of cloning. He said his position had once effectively been "pro-choice."
But Giuliani, who said he personally hates abortion, hedged when asked about his current position. "I think the Court has to make that decision and then the country can deal with it," he said. "... The states could then make their own decisions."
Alone among the top three contenders, Arizona Sen. John McCain has a career-long record of opposition to abortion.
The 10 rivals showed their conservative credentials across 90 minutes of debate at the Ronald Reagan Presidential Library, each claiming to be a worthy heir to the political legacy of the late 40th president.
They stressed the importance of persisting in Iraq, called for lower taxes and a muscular defense and supported spending restraint.
"The first pork barrel, earmark bill that crosses my desk I'm going to veto it and I'm going to make the author famous," said McCain.
Romney jumped in at that, saying that as governor he had cast a veto "hundreds of times." Former Wisconsin Gov. Tommy Thompson put his total at some 1,900 vetoes.
The field split on another issue, with Brownback, former Arkansas Gov. Mike Huckabee and Colorado Rep. Tom Tancredo raising their hands when moderator Chris Matthews asked who did not believe in evolution.
Democratic National Committee chairman Howard Dean said the debate "confirms that a Democrat will be elected in 2008. The Republican presidential contenders are only offering more of the same failed leadership and misplaced priorities that President Bush brought to the White House."
Giuliani, McCain and Romney were the first among 10 equals on the debate stage - the men with the most money and the best approval ratings in the polls more than eight months before the first 2008 national convention delegates are selected.
Other participants included former Gov. Jim Gilmore of Virginia; and Reps. Duncan Hunter of California and Ron Paul of Texas.
They debated in the shadow of Reagan's Air Force One, the aircraft hanging suspended in the library's pavilion. The 40th president's widow, Nancy Reagan, sat in the front row next to California Gov. Arnold Schwarzenegger. One by one, the candidates invoked Ronald Reagan - he was mentioned 19 times. The issue of abortion looms large in the 2008 presidential campaign in a party where a wide swath of political activists support the overturning of the 1973 Roe v. Wade ruling.
Both Romney and Giuliani must persuade conservative voters they are ready to embrace that view - or else persuade them to overlook the issue in picking a candidate for the White House.
In a debate that ranged broadly, most of the contenders said they opposed legislation making federal funds available for a wider range of embryonic stem cell research. The technique necessarily involves the destruction of a human embryo, and is opposed by many anti-abortion conservatives as a result.
There are exceptions, though, including Reagan's widow, Nancy. Also, public opinion polls show overwhelming support for the research, which doctors say holds promise for treatment or even cures of numerous diseases.
Most of the contenders said they opposed expanded federal research. McCain alone among the candidates said he unambiguously he supports expanded federal research into embryonic stem cells.
Thompson said there was "so much research" in the area that he couldn't respond yes or no.
Giuliani's response was open to interpretation. He said he supports it "as long as we're not creating life in order to destroy it," then added he would back funding for research along the lines of legislation pending in Congress.
The bill he cited does not expand research on embryonic stem cells, however, but deals with adult stem cells.
There was no dissent about the importance of the U.S. military mission in Iraq.
"We should never retreat in the face of terrorism," said Giuliani, adding, "terrible mistake."
Romney also said the United States must support the government of Nouri al-Maliki in its efforts to combat terrorism.
"I want to get our troops home as soon as we possibly can, but at the same time we don't want to get them out in such a precipitous way that we have to go back," he said, warning that too hasty a departure could lead to chaos in the region.
McCain said the war effort is now on the right track, although he said that until recently, the war had been "terribly mismanaged" by the Bush administration. "Terribly mismanaged," he repeated for emphasis.
The Iraq comments contrasted sharply with last week's debate among Democratic presidential hopefuls, where eight candidates called for an end to the military involvement that so far has claimed the lives of more than 3,300 U.S. troops.
Speaking of Iran, Giuliani said "they looked in Ronald Reagan's eyes and in two minutes they released the hostages." That was a reference to the U.S. hostages released from captivity on the day of Reagan's inauguration in 1981. He didn't mention other hostages taken on Reagan's watch - those seized in Lebanon and kept for years. Romney invoked Reagan in discussing abortion rights. "I changed my mind. I took the same course that Ronald Reagan and George Herbert Walker Bush" did, he said. Romney and McCain squared off over terrorist leader Osama bin Laden without directly addressing each other. Last week, the ex-governor said, "it's not worth moving heaven and earth spending billions of dollars just trying to catch one person" and advocated a broader strategy to defeat Islamic jihadists. McCain had called the comment "naive." Under questioning, Romney defended his comment, saying: "It's more than Osama bin Laden. But he is going to pay and he will die." McCain shot back, saying bin Laden's responsible for the deaths of thousands of innocent Americans. "We will do whatever is necessary. We will track him down. We will catch him. We will bring him to justice and I'll follow him to the gates of hell," he said. MSNBC and The Politico co-sponsored the debate.