WASHINGTON (Reuters) - Many supporters of Barack Obama hoped his election as America's first black president might herald an era of post-racial politics, but race has been an issue his administration just can't seem to avoid.

Division and tension between black and white Americans has cropped up repeatedly over Obama's 18 months in office, hurting his popularity and distracting from his political agenda.

The issue surfaced this week when the Agriculture Department pushed a black official to resign after allegations she discriminated against a white farmer, only to apologize a day later for acting too quickly and without the facts.

Some said the White House was too eager to prove to its critics on the right that it does not favor blacks.

"The Obama administration lost some political capital because they acted without thinking things through," said Andra Gillespie, a political scientist at Emory University.

Obama and race relations have often grabbed headlines.

Last July -- in the heat of the White House fight for its healthcare overhaul -- when Obama was subjected to scathing criticism for saying police had "acted stupidly" when they arrested Harvard University scholar Henry Louis Gates, who is black, on charges he was breaking into his own home.

More recently, the Justice Department dismissed voter intimidation charges against the New Black Panther Party, prompting criticism from conservative groups who said the black president was unwilling to prosecute fellow blacks for civil rights violations.

"When the right-wing noise machine starts promoting another alleged scandal, you shouldn't suspect that it's fake -- you should presume that it's fake, until further evidence becomes available," columnist Paul Krugman wrote in The New York Times.

CIVIL RIGHTS LAWS

It has been more than 40 years since landmark U.S. civil rights laws banned discrimination against blacks. But race in America remains a forceful, divisive factor in areas from jobs to educational opportunity to banking and home ownership.

Blacks account for 13 percent of the U.S. population and on average earn less and are more likely to be unemployed than other racial groups. They are also more likely to be arrested and are given harsher sentences.

"Racial conflict is America's deepest wound, still poorly healed," columnist Michael Gerson wrote in Wednesday's Washington Post.

Both the right and left accuse each other of injecting race into the political discourse. Experts say that's inevitable given Obama's position as the first non-white U.S. president. Obama's father was Kenyan and his mother a white American.

This week, Shirley Sherrod, a black official at the Agriculture Department, said her bosses pushed her to quit after conservative media repeatedly broadcast a tape that seemed to show her saying she had discriminated against a white farmer because of his race.

It was later found that the tape had been altered to misrepresent Sherrod's remarks at a meeting of the National Association for the Advancement of Colored People (NAACP), the oldest black rights advocacy group. She had in fact been saying that race should not matter.

Agriculture Secretary Tom Vilsack publicly apologized and the department offered her another job. She said she had not decided whether or not to take it.

DISTRACTION

Whether intended or not, the furor over the Sherrod case distracted media attention on Wednesday from one of Obama's biggest achievements -- his signing of a historic reform of financial regulation that was opposed by conservatives.

Conservatives had linked the tape to the NAACP asking the conservative "Tea Party" political movement to denounce racism by some of its members. Images such as Obama with a bone through his nose and the White House with a lawn full of watermelons are often displayed at Tea Party rallies.

Tea party leaders say the movement is not racist but concede there are racist fringe elements in its membership.

Gillespie said the stakes are higher for Obama because his presidential campaign sought to emphasize that it was would not be bogged down in racial disputes.

"So when Barack Obama got elected ... this meant that he wasn't 'supposed' to address racial issues and that if he did discuss racial issues, there would be a whole backlash," Gillespie said.

Such issues are a distraction from serious problems like racial disparities in U.S. unemployment and education, he said.

(Editing by Kristin Roberts and David Storey)