WASHINGTON (CNN) -- Sen. Hillary Clinton's lead over Sen. Barack Obama, her chief rival for the Democratic presidential nomination, is growing among African-American voters who are registered Democrats, and particularly among black women, a poll said Wednesday.

Sen. Hillary Clinton is the top choice of African-American Democrats, a new poll suggests.

Among black registered Democrats overall, Clinton had a 57 percent to 33 percent lead over Obama.

That's up from 53 percent for Clinton and 36 percent for Obama in a poll carried out in April.

The 26-point difference between black women and men underscores the fact that the nation's vote is divided not only by race, but also by gender, said CNN political analyst Bill Schneider. "Black women don't just vote their black identity," he said. "They also vote their identity as women."

Among white registered Democrats, Clinton drew 49 percent support, versus 18 percent for Obama and 17 percent for former Sen. John Edwards of North Carolina, the latest poll found.

The question had a sampling error of plus-or-minus 6.5 percentage points.

The former first lady's strongest support among blacks came from black women, 68 percent of whom identified her as their likely choice, versus 25 percent who cited Obama, the senator from Illinois who is African-American. Watch Bill Schneider examine CNN poll results on black political support »

Black men who are registered Democrats were nearly evenly split, with 42 percent favoring Clinton and 46 percent favoring Obama. The sampling error of that question was plus-or-minus 8 percentage points.

Black registered Democrats also appeared more sure of themselves than did whites, with two-thirds (67 percent) of blacks saying they would definitely support whichever candidate they had said they favored, versus one-third (33 percent) who said they might change their minds.

White registered Democrats appeared more open to persuasion, with only 45 percent saying they would definitely support that person, and 54 percent saying they might change their minds.

Overall, registered white voters polled appeared nearly evenly split between the two major parties, with 45 percent saying they would definitely or probably vote for the Democratic presidential candidate, and 47 percent saying the same about the Republican presidential candidate.

That question had a sampling error of plus-or-minus 6 percentage points.

But black voters appeared overwhelmingly likely to vote Democratic, with 80 percent of them saying they were definitely or probably going to cast their vote for the party's candidate.

Just 13 percent of blacks said the same thing about the Republican candidate.

Clinton maintained an overwhelming lead among black registered voters when pitted against former New York Mayor Rudy Giuliani, besting the leading GOP presidential candidate 86 percent to 13 percent.

But when the same question was asked of white registered voters, the senator from New York's lead vanished, with Giuliani outpolling her 57 percent to 40 percent.

That question had a sampling error of plus-or-minus 6 percentage points.

The questions were asked based on the absence of former Vice President Al Gore from the race. He has repeatedly said he has no plans to run.

But, were he to change his mind, and if he won the Democratic nomination, the poll found that 52 percent of 927 registered voters queried said they would choose him versus 46 percent who cited Giuliani.

Among blacks, that lead was 78 percent for Gore versus 20 percent for Giuliani.

In response to poll results showing him trailing Clinton, Obama has noted that Clinton has been a major figure in national politics for 15 years, versus just three for Obama.

"The 'sistah' vote is paying off handsomely for Hillary Clinton," said Democratic political strategist Donna Brazile. "It's not only getting her the women's vote. It's also getting her the black vote."

A gender gap also exists among white Democrats, but -- at 11 points -- it is narrower.

The CNN/Opinion Research Poll of 1,212 adult Americans was carried out by telephone Friday through Sunday. The sample included interviews of 307 blacks and 762 non-Latino whites. E-mail to a friend

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