WASHINGTON — Landslide margins among black voters powered Barack Obama to his win yesterday in South Carolina's Democratic presidential primary, allowing him to overcome the telling edge Hillary Rodham Clinton and John Edwards had among whites.

Blacks made up 55 percent of the voters in yesterday's contest, slightly more than turned out in the state's primary four years ago and by far their biggest share in any presidential contest so far this year. Obama won 78 percent of their votes, with black men and women supporting the Illinois senator by about that same margin, according to exit polls of Democratic voters conducted yesterday for The Associated Press and the networks.

Clinton and Edwards split the white vote about equally, with each getting support from nearly four in 10 and Obama getting about a quarter. Obama's high-water mark among white voters so far this year has been the 36 percent he got in New Hampshire, where he finished second overall to Clinton; he also got a third of the white vote in the year's first contest in Iowa, enough for him to win overall in that state.

About six in 10 of Clinton's and nearly all of Edwards' votes came from whites.

Racial attitudes also were in play in voters' perceptions of how effective the candidates would be if elected. Whites were far likelier to name Clinton than Obama as being most qualified to be commander in chief, likeliest to unite the country and most apt to capture the White House in November. Blacks named Obama over Clinton by even stronger margins — 2- and 3-to-1 — in all three areas.

After a week of criticism between the Obama and Clinton campaigns in which race became a factor, Obama's relatively small share of white supporters in South Carolina could raise questions about his ability to attract those voters in the crucial Super Tuesday contests on Feb. 5, when nearly half the country will vote.

Because of his heavy support from blacks, Obama negated the advantage Clinton has enjoyed among women in most of this year's contests. He got more than half the female vote, compared with three in 10 supporting Clinton, the New York senator.

But the gender breakdown was heavily affected by race. Though Obama won eight in 10 votes of black females, Clinton and Edwards led among white women, getting about four in 10 of their votes, about double Obama's share.

Edwards, the former North Carolina senator, led among white men, garnering about four in 10 of their votes, with Clinton and Obama about equally sharing the rest.

In a race featuring candidates who would be the country's first female or first black president, about three-quarters of Democrats said they thought the country was ready for either historic event to occur.

Bill Clinton's campaigning in the state — in which he engaged in some of the campaign's sharpest attacks on Obama — was cited as an important factor by nearly six in 10 voters, including about equal amounts of blacks and whites.

As has been the pattern in most of the Democratic contests this year, the economy was cited as the most important issue facing the nation by far, with about half naming it.

In another replay of Democratic sentiment from other states' voting, about half said they wanted a candidate who can bring change, making it the most sought-after quality. And once again this was dominated by Obama, who has made it the leading theme of his campaign, as he won three in four voters who named it.

The poll was conducted by Edison Media Research and Mitofsky International. The poll interviewed 1,905 Democratic primary voters and had a margin of sampling error of plus or minus 3 percentage points.