But Obama's campaign saw the limits of black support in last week's losses in Ohio and Texas, which kept Clinton's campaign alive. And the role black voters will play in the next big contest, Pennsylvania's April 22 primary, is unclear.
Moreover, some analysts think it's possible Obama's heavy black support is nudging some working-class white Democrats into Clinton's camp. If true, it could be an important factor in a contest that remains remarkably tight after a year of campaigning.
Obama, the son of a white mother from Kansas and a black father from Kenya, won slightly more white votes than Clinton in Wisconsin, Virginia and a few other states last month, helping him to a string of wins and the overall lead in delegates to the party's national convention.
But Clinton won nearly two out of every three white votes in Ohio, and 56 percent of those in Texas, where she also ran well among Latinos. Strategists are pondering the results, wondering if Pennsylvania's demographic similarities to Ohio will deliver another important win to Clinton in six weeks.
Ronald Walters, a University of Maryland political scientist who tracks racial trends and is writing a book on Obama, thinks Obama's strong support from blacks made it easier for some whites in Ohio and Texas to vote for Clinton.
"There's some of that," Walters said in an interview. He pointed to exit polls from Ohio, where 62 percent of all whites lack college degrees and many are anxious about their jobs in a weak economy.
"This is a racially sensitive group," he said, referring specifically to whites who earn less than $50,000 a year and did not attend college.
"They are the quintessential Reagan Democrats," he said. "They feel they've been left" and their resentment can have social and racial overtones.
In general elections, which pit Democrats against Republicans, the racial sensitivity of white voters has been pronounced and well-documented for decades. It's a chief cause of the realignment of the South, where blacks remained intensely loyal to the Democratic Party as whites moved to the GOP by the millions.
In the intraparty world of Democratic primaries, however, racial divisions are much less prevalent, and hard to measure. Many white Democrats, especially in the South, tend to be liberal, racially tolerant and usually happy to join blacks in opposing Republicans.
The Obama-Clinton rivalry may be straining that comity. Some blacks resented remarks Clinton made in New Hampshire, which they viewed as minimizing Martin Luther King Jr.'s role in achieving landmark civil rights laws. And after Obama's South Carolina victory on Jan. 26, former President Clinton seemed to equate the Illinois senator with Jesse Jackson as a candidate who could not draw widespread white support.
Many blacks felt the Clintons "were trying to use race to their political advantage, to cede the black vote to Obama and take the rest," said David Bositis, senior political analyst for the Joint Center for Political and Economic Studies, which tracks issues important to black Americans.
The Clintons said they intended no slights, and many blacks still hold great affection for the former president and his eight-year term. But Hillary Clinton's sharp-elbowed campaign is alienating others, Bositis said, and it could hurt the New York senator in November if she becomes the nominee.
Bositis said it was unclear whether Obama's black support is driving some working-class whites into Clinton's corner, but he noted the steep drop in Obama's share of the white vote in Ohio compared to Wisconsin. One possible factor other than race, Bositis said, was Clinton's strong support within the Ohio Democratic establishment, starting with the governor.
One thing is not in doubt: Obama's candidacy and the closeness of the contest are triggering record turnout among black voters. "In many states, the black vote has doubled," Bositis said.
Similar turnout in Philadelphia's black neighborhoods could help Obama next month. But he would have to make deeper inroads into Pennsylvania's white electorate than he did in Ohio if he is to avoid another solid defeat.
Meanwhile, Clinton continues to draw about 10 percent to 20 percent of black voters, who sometimes have to defend their choice.
"She has the most experience," said Elexis Griffin, a black worker at a law office who attended a Clinton fundraiser in Canton, Ohio. "Obama has only been in the Senate three years. I'm not anti-Barack. I'm just pro-Hillary."
Griffin, who is 25 and considering law school, said, "I sit here almost every single day and hear debating: Hillary or Obama? My closest friends, I have very much influenced their vote for Hillary. They accuse me of being against the social movement. And I accuse them of voting with their emotions and not looking at the facts."