Racial prejudice in America is more widespread now than when President Barack Obama became the country's first black president in a historic 2008 vote, a new survey has shown.

In a poll of racial attitudes by the Associated Press news agency, researchers found that more Americans have attitudes that are both implicit and explicitly racist than when the same survey was conducted four years ago.

The news comes as Obama is deadlocked in a tight race for re-election against Republican challenger Mitt Romney and surveys have shown strong support for Obama among minorities while white voters favour Romney.

In all, 51% of Americans now express explicit anti-black attitudes, compared with 48% four years ago, the study showed.

When measured by an implicit racial attitudes test, the number of Americans with anti-black sentiments jumped to 56%, up from 49% during the last presidential election.

"As much as we'd hope the impact of race would decline over time … it appears the impact of anti-black sentiment on voting is about the same as it was four years ago," said Jon Krosnick, a Stanford University professor who worked with AP to develop the survey.

A majority of Americans expressed anti-Hispanic sentiments, too.

In an AP survey done in 2011, 52% of non-Hispanic whites expressed anti-Hispanic attitudes. That figure had risen to 57% in the implicit test in 2012.

There was no comparative data for before that. Though race has not been played an especially high profile in the election campaign so far America, like many societies, still struggles with racism.

During his four years in office Obama has repeatedly had to contend with untrue rumours that he is a Muslim or was not born in America – a phenomenon of fear of "the other" that some link to his being a black American.

The AP poll showed that racial prejudice against Obama could cost him five points in the current election campaign.

However, conversely pro-black sentiment could see him scoop an extra three points, meaning a net loss due to racial attitudes of 2%.

The survey also found that some 79% of Republicans expressed explicit racial prejudice compared to 32% of Democrats.

The implicit test too showed that 64% of Republicans had racial prejudice compared to 55% of Democrats, while independents came in at 49%.

The explicit racism test asked respondents whether they agreed or disagreed with a series of statements about black and Hispanic people.

In addition, the surveys asked how well respondents thought certain words, such as "friendly", 'hardworking", ''violent" and "lazy" described blacks, whites and Hispanics.

The same respondents were also administered a survey designed to measure implicit racism, in which a photo of a black, Hispanic or white male flashed on the screen before a neutral image of a Chinese character.

The respondents were then asked to rate their feelings toward the Chinese character. Previous research has shown that people transfer their feelings about the photo onto the character, allowing researchers to measure racist feelings even if a respondent does not acknowledge them.

The AP poll comes amid a slew of books and research studies analysing the current situation of black America that have found it has suffered greatly in the wake of the Great Recession.

The Pew Charitable Trust showed that some 68% of middle class black Americans are predicted to see their economic status decline in the next generation.

The National Urban League civil rights group also showed from 2009 to 2012 median annual household income for blacks dropped by 11.1%, compared to a drop of just 5.2% for whites and 4.1% for Hispanics.

The current black unemployment rate of 14% is roughly twice that of the white jobless rate.

Elsewhere ,a new book, called Documenting Desegregation, has examined racial equality in the workplace after the 1960s.

It found that progress largely halted in 1980 and in fact has actually got worse in some industries since then.

Indeed racial segregation between white and black men is increasing in one-in-six industries.

Co-author Donald Tomaskovic-Devey, a professor at the University of Massachusetts, said societal attitudes had changed in America, but not practical applications.

"Most white Americans do think that we should live in an equal opportunity society. But we do not actually apply that very practically. On a behavioural level, we are not post-racial," he said.