April 12 (Bloomberg) -- Former Senator Fred Thompson, who has yet to announce his candidacy, is ahead of a slumping John McCain, and Barack Obama is closing in on Hillary Clinton, according to the latest Bloomberg/Los Angeles Times poll of Americans' 2008 presidential preferences.

The Iraq war dominates voters' concerns to the detriment of Republicans, the April 5-9 poll suggests. A plurality of voters wants the Democrats to take the presidency next year, and most Republicans say they want their party's nominee to move the country in a different direction than President George W. Bush.

In head-to-head general-election match-ups, the poll shows Obama, an Illinois senator, is the strongest Democrat, beating all Republicans, and Senator Clinton, of New York, appears to be the weakest. Among the leading Republicans, excluding Thompson, former New York Mayor Rudy Giuliani runs best and former Massachusetts Governor Mitt Romney runs worst.

``Republicans are starting out in a net negative'' and ``the Iraq war is playing a large part,'' says Susan Pinkus, the Los Angeles Times polling director.

In the Republican field, 29 percent of voters prefer Giuliani while McCain, an Arizona senator who once led the pack, slips to 12 percent, behind Thompson, who is preferred by 15 percent and said last month that he is considering a run for the presidency. Thompson, 64, disclosed yesterday that he has been treated for indolent lymphoma, a slow-growing form of the cancer, and the disease is in remission.

`No Illness'

``I have had no illness from it, or even any symptoms,'' Thompson, an actor and former Tennessee senator who has a long- running role in the NBC television series ``Law and Order,'' wrote in a posting on the Web site Redstate.com.

Thompson's instant popularity suggests he has filled a void in the Republican field and his potential candidacy takes from all of the Republican candidates, the poll finds. He does especially well with self-described religious Republicans; 21 percent of those voters favor Thompson, compared with 17 percent for Giuliani, 62, and 10 percent for McCain, 70.

During his eight years in the Senate, Thompson was never identified with religious conservative causes.

Poll respondent Thomas Tobolski, a 61-year-old Republican computer analyst from South Bend, Indiana, says he likes Thompson because he wants more choices. ``I'm looking for somebody who's a little more in the conservative vein,'' he says.

Giuliani Leads Republicans

Giuliani remains far ahead when he is matched in a three-way race against McCain and Romney, defeating McCain by 23 points and Romney by 28 points. The poll includes 1,246 registered voters, among them 557 Democratic primary voters and 437 Republican primary voters. The survey has a 4 percentage-point margin of error for Democrats and a 5-point margin for Republicans.

McCain, who over the past year has consistently supported Bush and the administration's Iraq policies, has had a substantial decline in support and trails in fundraising.

The poll was taken before McCain made a speech yesterday on the war that tightened his association with Bush's war policy. McCain reiterated his support for putting more troops into the conflict and said he would hold to his stance even if it cost him his bid for the White House. McCain recently returned from his fifth visit to Iraq and cited several areas of progress in an opinion article this month in the Washington Post.

The poll shows that McCain's support for Bush, 60, and the war hasn't worked so far with Republicans. Twice as many Republicans who believe the president's war plan is helping Iraq support Giuliani over McCain. Republican voters who say they want the next president to continue Bush's policies -- about a third of the party -- overwhelmingly prefer Giuliani or Thompson, with McCain a distant fourth.

Age Factor

McCain, who will be 72 in 2008, may have another problem. Fourteen percent of all voters say they couldn't vote for a candidate who is that age, even if they agree on most issues.

For candidates of both parties, age is a greater impediment to the nomination than other personal factors, including Romney's Mormon religion, Giuliani's three marriages, Clinton's gender, and race for Obama, who is African-American.

Romney, 60, who raised more money than any other Republican in the first quarter, is in single digits, with just 8 percent of Republican voters saying they would support him. Eleven percent of all voters say they couldn't vote for a Mormon.

Democratic Match-Ups

On the Democratic side, Clinton, at 33 percent, is 10 points ahead of Obama. A Gallup poll last week showed a much larger lead. Former North Carolina Senator John Edwards and former Vice President Al Gore trail at 14 and 13 percent, respectively. All other Democratic hopefuls barely register. When the list is narrowed to Clinton, Obama and Edwards, the gap between the candidates stays about the same, with Clinton beating Obama, 42 percent to 32 percent.

In a three-way race, Clinton, 59, has an advantage over Obama, 45, both with blacks, who prefer her 50 percent to 41 percent, and women, who choose her by a 15-point margin.

Still, Obama is proving competitive with Clinton, and the poll shows he fares better among young voters. He also has a strong following in the Midwest, where he is behind Clinton by just 4 points.

``The main thing is his charisma and his ability to relate to people my age,'' says poll respondent Brian Kirn, an 18-year- old high school student in St. Louis. ``He just gives me a general feeling he's someone I could trust with the country.''

Anti-War Base

The anti-Iraq war base in the Democratic Party also offers opportunity for Obama and Edwards, who were quicker than Clinton to call for a timetable for withdrawal. Among the 37 percent of Democrats who say a timetable will help U.S. troops, Clinton only holds a narrow lead over Obama, with Edwards a solid third. Among Democrats who think a timetable hurts troops -- more than a third -- Clinton has a sizeable lead over both.

Edwards, 53, is targeting more traditional Democratic groups, including unions, with a populist message on worker rights, health care and other issues. Still, he gets more support from Democrats who consider themselves moderates than from those who say they are liberals, by 20 percent to 10 percent.

Gore, 59, who was the Democratic nominee in 2000 and has said he doesn't plan to run next year, is about even with Edwards, at 13 percent.

In head-to-head match-ups, the Democratic candidates generally do better, except against Giuliani, whose lead over Clinton and Edwards is within the poll's margin of error.

Clinton is 6 points behind Giuliani, and Obama narrowly leads Giuliani by 4 points. Obama is also ahead of McCain by 8 points and Romney by 19 points, compared with Clinton's 3-point lead over McCain and 7-point advantage over Romney.

When it comes to the issues, most Democratic voters, 63 percent, say the Iraq war is the most important topic they want the candidates to address, with health care and economic growth far behind.

Among Republicans, 37 percent say the war is the most important issue, while 28 percent choose economic growth and 13 percent pick tougher laws on illegal immigration.

To contact the reporter on this story: Heidi Przybyla in Washington at hprzybyla@bloomberg.net

Last Updated: April 11, 2007 18:40 EDT