WASHINGTON — One in three Republican voters favors withdrawing U.S. troops from Iraq, according to a new survey, a growing trend that could shape the 2008 presidential contest and lead more Republican lawmakers in Congress to abandon support for President Bush's troop buildup.

While only 7 percent of Republican voters want troops withdrawn immediately, 27 percent think that the Bush administration needs to set some "timeline" for withdrawal, once a taboo word in party circles. Still, 63 percent remain opposed to any timeline. Fabrizio, McLaughlin and Associates, a Republican research firm, conducted the survey of 2,000 self-described Republicans from May 28 to June 3.

The survey also found that Republican voters are most concerned with national security and foreign affairs (28 percent), while morality issues fell to second place (24 percent).

That could help explain why former New York Mayor Rudy Giuliani, a twice-divorced abortion-rights supporter known for his leadership after the terrorist attacks of Sept. 11, 2001, led the pack of Republican hopefuls among each of seven subgroups of Republican voters identified in the survey, including "moralists."

Giuliani was trailed by Arizona Sen. John McCain, actor and former Sen. Fred Thompson of Tennessee and former Massachusetts Gov. Mitt Romney. Nearly 1 in 5 would consider supporting billionaire New York Mayor Michael Bloomberg if he runs as an independent.

"Iraq is going to have a huge impact on the 2008 field," said pollster Tony Fabrizio, who was chief pollster to Republican Bob Dole's 1996 presidential campaign and isn't polling for any current candidate. "The question is going to become, how deftly can the nominee kind of dance around 'no timeline' without seeming inflexible?"

The out-of-Iraq sentiment was bolstered this week when two respected Republican senators, Richard Lugar of Indiana and George Voinovich of Ohio, called publicly for Bush to change course. A crucial juncture for Republican lawmakers could come in September, when Congress gets a progress report on the troop increase.

"You still have a majority opinion in the party that there is no timeline, (but) prior to even finding out the impact of the surge, you've got more than a third of Republicans who want to set some type of deadline," Fabrizio said. "You can clearly see more Republicans breaking ranks with the president, and the more who break ranks, the greater likelihood there could be a congressional mandate to do so."

The survey questioned half of its participants by telephone and half online from a geographically representative sample, with a margin of error of plus or minus 2.2 percentage points. It was partially underwritten by four socially moderate-to-liberal Republican organizations.

Some other findings:

_ Three-fourths said they could change their minds about whom they'll support for president.

_ More Republicans consider themselves "conservative" than Republicans did in a comparable survey that Fabrizio directed a decade ago; 71 percent, up from 55 percent.

But that label doesn't always fit stereotypes. One-third said the government wasn't doing enough about global warming, while 22 percent said the government was doing too much. Half said universal health care should be a guaranteed right for all Americans.

_ Two-thirds would consider electing a president with whom they disagreed on abortion policy.

_ Republicans are getting older. A comparable survey by Fabrizio's firm a decade ago found that 28 percent were 55 or older. In the new survey, 41 percent were 55 or older.

_ A decade ago, the central interest of about half of Republican voters was the economy. Only 16 percent said that now.

_ By an overwhelming gap — 71 percent to 16 percent — Republicans said they'd be more likely to support candidates in the mold of President Reagan than of President George W. Bush.