SIMI VALLEY, Calif. - Ten Republican presidential candidates wanting to succeed President Bush embraced a more popular president, conservative icon Ronald Reagan, at every turn in their first debate of the 2008 race.

"Ronald Reagan was a president of strength," Mitt Romney intoned. "Ronald Reagan used to say, we spend money like a drunken sailor," said John McCain. And Rudy Giuliani praised "that Ronald Reagan optimism."

The world, however, is far different than it was 25 years ago.

Iraq and terrorism now are top issues, support for Bush is at a low point, and Republican candidates find themselves trying to prove to the party's base that they are conservative enough to be the GOP nominee - on social matters as well as the economic and security issues Reagan championed.

The three leading candidates - Giuliani, McCain and Romney - and their seven lesser-known rivals attempted to do just that Thursday at the Ronald Reagan Presidential Library.

One by one, they invoked Reagan 19 times. In contrast, Bush's name was barely uttered.

"They went out of their way on multiple occasions, no matter the question, to associate themselves with Reagan," said Mitchell McKinney, a political communication professor at the University of Missouri-Columbia. "They tried their best to not be explicitly bashing or attacking Bush."

Republican operatives agreed that the debate did nothing to shake up the crowded GOP field.

They said Giuliani, the former New York City mayor, McCain, a four-term Arizona senator, and Romney, the ex-Massachusetts governor, remained the strongest contenders.

"Clearly, the top three looked quite presidential," said Whit Ayres, a Republican pollster.

Scott Reed, who ran Bob Dole's 1996 campaign, added: "McCain showed a little energy. Romney showed he's very polished. And Giuliani started to clear up some of his issues with the base of the party."

Others, however, questioned whether Giuliani had begun to clear the matter up.

Each largely stuck to his talking points as he tried to present himself as the most conservative candidate in the pack, and a worthy heir to Reagan.

The candidates expressed resolve in winning the war in Iraq and defeating terrorists across the world. They also had to answer for their positions on a range of social issues, including abortion, stem-cell research and evolution.

"Nobody wants to talk about social issues for more than 11 seconds," said Rich Galen, a GOP strategist. "But they had to talk about what they were asked about."

McCain is the only top-tier contender who has a career-long record of opposing abortion.

With a record of supporting abortion rights, Giuliani was the only candidate who said "it would be OK" if the Supreme Court upheld the landmark ruling. "It would be OK to repeal it. It would be OK also if a strict constructionist viewed it as precedent," he said.

His rivals agreed that it would be a great day if the court overturned the landmark ruling.

Romney, for his part, acknowledged he had reversed course on the subject and said his position had once effectively been "pro-choice."

"I changed my mind," Romney said, adding that Reagan did the same.

Most of the contenders said they opposed legislation making federal funds available for a wider range of embryonic stem-cell research.

McCain was the only one to unambiguously say he supports expanded federal research into embryonic stem cells.

Giuliani's response was open to interpretation. He said he supported 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 increase federal support for research on embryonic stem cells. Rather, it deals with adult stem cells.

The field split on another issue, with Sen. Sam Brownback of Kansas, former Arkansas Gov. Mike Huckabee and Colorado Rep. Tom Tancredo raising their hands when asked who did not believe in evolution.

The other participants were former Govs. Jim Gilmore of Virginia and Tommy Thompson of Wisconsin, and Reps. Duncan Hunter of California and Ron Paul of Texas.