WASHINGTON — The first Republican presidential debate previewed a campaign ahead in which Mayor Giuliani will say the party needs to reach out to Democrats and independents so it can win while a tier of lesser-known lawmakers and former governors portray themselves to Republican primary voters as “reliable” and “consistent” conservatives.

The debate illuminated differences between most of the field and Mr. Giuliani, who faced repeated questions about his position on abortion. The former mayor sought a nuanced view, saying early on that it would be “okay” if the Supreme Court overturned the landmark Roe v. Wade decision; later in the night he stated clearly that he supports a woman’s right to terminate a pregnancy. He said multiple times that he “hated” abortion, calling it a “very, very difficult matter of conscience.”

“Ultimately, since it is a matter of conscience, I would respect a woman’s right to make a different choice,” he said.

Mr. Giuliani was the only candidate to hedge on whether the overturning of Roe v. Wade would be positive, and several candidates went to great lengths to proclaim their support for “life.”

Senator Brownback of Kansas said it would be a “glorious” day, while a former Arkansas governor, Michael Huckabee, said the issue of life was neither insignificant nor “small.”

Mr. Brownback, who is known for his social conservatism, also made a plea for the return of faith to public life.

“We shouldn’t be running faith out of the public square,” he said. “We should be inviting it in and celebrating it.” Mr. Giuliani sought the political center at times. With an eye toward a general election, he said the party had to “reach out, bring in Democrats, bring in independents.” The debate also spotlighted differences on immigration. Mr. Giuliani used his first chance to speak to hint at a moderate position, citing it as a strength to build on. And when the candidates were asked if they supported a constitutional amendment allowing non-native citizens to become president, Mr. Giuliani said he would be open to it, in contrast to most of his rivals. Seated in the front row was the Austrian-born California governor, Arnold Schwarzenegger.

At the opposite end was a Colorado congressman, Rep. Tom Tancredo, who chastised Republicans who said they were not for “amnesty” even as they supported a path to citizenship for an estimated 12 million illegal immigrants now living in America.

On Iraq, there was agreement by all but one of the candidates in opposing a withdrawal of American troops. The lone exception was a Texas congressman, Rep. Ron Paul, who advocated a “policy of nonintervention” and said he opposed the Iraq war from the beginning.

A former Massachusetts governor, Mitt Romney, and Senator McCain of Arizona appeared set on one-upping each other on the issue of Osama bin Laden. Mr. McCain had criticized Mr. Romney for suggesting last week that catching the Al Qaeda leader was not important. Asked about it last night, Mr. Romney said “we’ll move everything” to capture him. “It’s more than Osama bin Laden, but he will pay, and he will die.” Mr. McCain, in turn, said he would follow Mr. bin Laden “through the gates of hell.”

Mr. McCain, whose candidacy has struggled in the early going, stumbled at times last night as he tried to deliver scripted responses. At one point, he continued talking well after the moderator cut him off, protesting that he thought his yellow warning light was still flashing.

The 90-minute forum was short on rancor between contenders and long on tributes to the late President Reagan, whose library in Simi Valley, Calif., played host to the first face-to-face debate of the Republican nominating contest. With Nancy Reagan seated in the front row, the candidates invoked the 40th president’s legacy dozens of times.

Perhaps the most awkward moment of the debate came when each candidate was asked if he supported embryonic stem cell research, a cause that Mrs. Reagan champions but social conservatives oppose. Most of the candidates said they opposed the method; Mr. McCain said he supported it, and Mr. Giuliani said he supported it as well, but with limitations. Entering last night’s debate, there was no doubt that most if not all of the Republican hopefuls would try to link themselves to Reagan, but an open question was how far the candidates would stray from President Bush, whose approval ratings have lingered in the 30s. The contenders appeared hesitant to criticize the sitting president by name, but Mr. McCain went the furthest, assailing federal spending and saying the Iraq war was “terribly mismanaged.” Mr. Giuliani, however, went out of his way to praise Mr. Bush for his conduct in the war on terrorism, highlighting the fact that there have been no attacks since September 11, 2001.