WASHINGTON — Sen. John McCain, once considered the front-runner for the Republican presidential nomination, has fallen to third place in a new Los Angeles Times/Bloomberg poll, and is running behind Fred Thompson, an actor and former senator who has not even entered the race.

Former New York Mayor Rudolph W. Giuliani leads the crowded field of announced and potential contenders with support from 29% of probable Republican primary voters surveyed, followed by Thompson with 15% and McCain with 12%. Mitt Romney, the former Massachusetts governor and a fundraising powerhouse, had 8%.

The Arizona senator's showing in the poll is his lowest in any national survey to date, marking a new benchmark in his flagging fortunes. The surge of interest in Thompson is a sign of conservative dissatisfaction with the established field of candidates and underscores just how unsettled the Republican race remains.

"Thompson is a Reagan conservative, and that's what I want," said Robert Little, a poll respondent in Duluth, Ga., who views other leading Republican candidates as unreliable allies on social issues.

Among probable Democratic primary voters, the campaign continues to solidify into a three-way race, with Sen. Hillary Rodham Clinton of New York leading Sen. Barack Obama of Illinois and former Sen. John Edwards of North Carolina by comfortable margins.

But the poll signals larger obstacles for Clinton in matchups against top Republicans: Even though voters overall said, by a 10-point margin, that they would prefer to see a Democrat win in 2008, the poll shows Clinton eking out only a narrow lead over McCain, and running behind Giuliani.

Contrasting contests

Overall, the poll paints contrasting portraits of the two parties' presidential contests at this early stage in the campaign.

Democrats have a well-settled field that is beginning to divide voters along class and income lines: Clinton is running strongest among lower-income voters, and Obama is besting her among higher-earning Democratic voters. Clinton is the favorite among black voters, even though Obama is himself African American.

The Republican candidate field, by contrast, is still in flux and ill-defined.

"Republicans, unlike Democrats, are not totally satisfied with their choices," said Karlyn Bowman, a public opinion analyst at the American Enterprise Institute. "Giuliani is the clear leader, but there's a lot of competition for second place."

Ready for change

The survey also shows significant Republican desire to move beyond the George W. Bush era: 61% of Republican voters surveyed said they wanted the next GOP nominee to campaign on a platform of moving in a new direction. Only 30% said the nominee should call for continuing Bush policies.

The poll, conducted last Thursday through Monday under the supervision of Times Poll Director Susan Pinkus, surveyed 1,373 adults, including 1,246 registered voters. It had a margin of error of plus or minus 3 percentage points.

The sample included 557 probable Democratic primary voters, who were asked about potential Democratic candidates, and 437 probable Republican primary voters, who were asked about GOP hopefuls. The margin of error was plus or minus 4 percentage points for Democrats, and plus or minus 5 points for Republicans.

Trouble for McCain

McCain's third-place ranking in the poll comes at a time when his campaign has been buffeted by trouble on many fronts. He raised less money in the first quarter of 2007 than did Giuliani or Romney. His vigorous support for the Iraq war has cost him support among independent voters who were crucial to his 2000 campaign. He drew heavy criticism last week for comments about the security situation in Baghdad that were widely seen as Pollyannaish.

As part of an effort to revive his campaign, McCain has inaugurated a series of major policy addresses. The first one was Wednesday, on his continuing support for the Iraq war.

When the survey reduced the field to three candidates, Giuliani's lead was more decisive: He drew 48% to McCain's 25% and Romney's 20%.