John Kerry (search) holds a double-digit lead over President Bush among likely voters in California, the Democrat's candidacy buoyed by strong support from independents.

A telephone survey of 647 registered voters who expect to go to the polls found Kerry favored over Bush in a hypothetical matchup, 55 percent to 40 percent, with 5 percent undecided, according to the nonpartisan Field Poll released Wednesday.

"Eight out of 10 Democrats prefer Kerry and a similar proportion of Republicans back Bush. The key to Kerry's overall lead is a result of independent, nonpartisan voters ... splitting more than 2-to-1 in Kerry's favor," the survey said.

The poll, conducted May 18-24, had a margin of error of plus or minus 4 percentage points.

Although Bush's approval ratings are at the lowest of his presidency because of the war in Iraq, Kerry's support in the state has barely advanced since February, when an earlier Field Poll of registered voters found showed a 53 percent to 41 percent lead over the president. Bush held the advantage in a January poll.

The poll also found that Bush's standing in the state was based on a desire to end his presidency, rather than an endorsement of Kerry. Sixty-five percent of Kerry supporters identified their votes as more a rejection of Bush than support for the presumptive Democratic nominee, the poll said.

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Kerry's image has improved in battleground states in which his campaign has launched millions of dollars in advertising to introduce him to voters and counter Bush's critical commercials.

A poll released Wednesday found that 44 percent of people in the battleground states view Kerry favorably. His standings have improved since early May, when 39 percent had a favorable impression, according to the University of Pennsylvania's National Annenberg Election Survey.

The Annenberg survey identified battleground states as Arizona, Arkansas, Colorado, Delaware, Florida, Iowa, Louisiana, Maine, Michigan, Missouri, Minnesota, Nevada, New Hampshire, New Mexico, Ohio, Oregon, Pennsylvania, Washington, West Virginia and Wisconsin.

The survey called 800 adults in those states from May 17-23. It had a margin of sampling error of plus or minus 3 percentage points.

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In Iowa, Kerry has a slight lead over Bush, 46 percent to 42 percent, with independent Ralph Nader (search) at 3 percent and 9 percent undecided, according to a poll released Wednesday.

The Research 2000 poll was done for KCCI-TV.

Kerry led Bush by 14 points among independent voters in a three-way race, and by about the same margin among independents in a two-way race. In a two-way matchup, Kerry led Bush 48 percent to 43 percent.

The poll of 604 likely voters was taken May 23-25 and has a margin of sampling error of plus or minus 4 percentage points.