WASHINGTON (CNN) -- The number of Americans who believe that the country is ready for a black president is rising, a poll released Thursday suggested.

Sen. Barack Obama campaigns Thursday in Philadelphia, Pennsylvania.

More than three quarters, 76 percent, of respondents in a CNN/Essence Magazine/Opinion Research Corp. poll said the country is ready to be led by an African-American, up 14 percentage points since December 2006.

Some of the rise can be attributed to the success of Sen. Barack Obama in the Democratic presidential primaries, said Keating Holland, CNN's polling director.

"We're not asking this question in a vacuum. In many cases, respondents must have had Obama in mind when giving their answer, even though he is not mentioned anywhere in the questionnaire," Holland said.

The senator from Illinois is locked in a battle with Sen. Hillary Clinton of New York for the Democratic presidential nomination. Obama leads Clinton in states won, in delegates pledged and in the overall popular vote in the primaries and caucuses held.

The poll also indicates that more whites than blacks think the country is ready for a black president. Of the white Americans surveyed, 78 percent said the country is ready, as opposed to 69 percent of African-Americans polled. Both numbers are up substantially from December 2006. Watch panel debate candidates' campaign strategies »

"Drawing on their own life experience, blacks are a little more skeptical than whites. But blacks, too, have come around, particularly after the Iowa caucuses demonstrated that Obama could win in an overwhelmingly white electorate," said Bill Schneider, CNN senior political analyst."Among blacks, the belief that the country is ready for an African-American president is highest among blacks who share traits with Obama," Holland said.

"Optimism about the country's acceptance of a black president is higher among black men than among black women, higher among college-educated blacks than among those with no college degree and higher among younger blacks than older blacks," Holland said.

The poll also suggested that more Americans think the country is ready for a black president than a female president. Sixty-three percent of those surveyed say the country is ready for a female president, 13 points lower than those who say the country is ready for a black president.

"Do Americans see more prejudice against a woman than an African-American? More likely, they see more negative feelings about this woman than about this African-American. Because it's true. More people have an unfavorable opinion of Hillary Clinton than of Barack Obama," Schneider said.

The poll asked whether the country is ready for a black or female president, not whether respondents would vote for a black or female president.

"Few people will acknowledge their own prejudices, but they will answer whether they think the country is ready to elect a black or woman president," Schneider said.

The poll was conducted by telephone from March 26 to April 2, with 2,184 Americans questioned, including 1,014 blacks and 1,001 whites.

The survey is being released on the eve of the 40th anniversary of the assassination of the Rev. Martin Luther King Jr.

Those polled were asked whether they felt that the U.S. has fulfilled King's vision spelled out in his "I have a dream" speech in Washington in 1963.

Thirty-four percent said yes; 41 percent said no but that they believe it's possible; and 19 percent said no, that it is impossible.

The survey's margin of error is plus or minus 2 percentage points for the overall sample and plus or minus 3 percentage points for the questions asked just to blacks or whites. E-mail to a friend

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