There's something not quite right about these Republican attack ads

Attack ads against Barack Obama in the 2008 presidential election race depicted him with darker skin which appealed to some voters' racial prejudices, a new study has alleged.

A study published online this month in Public Opinion Quarterly analysed 126 advertisements from the campaign, by digitally measuring the darkness of the two nominees' skin in each advert, and then sorted them into different categories.

In negative spots, both campaigns manipulated images of the opponent to shadow or wash out their face.

However, as the election approached, images of Obama in McCain’s campaign adverts became gradually darker, while McCain’s skin tone grew lighter - it is not known whether the move was deliberate.

The study also showed that presenting viewers with digitally manipulated, darker images of Obama did affect the way they perceived him.

Those who saw an image of him with darker skin associated him with an anti-black stereotype 45 per cent of the time, compared to 33 per cent with a lighter image.

The Washington Post reports that this purported tactic is sometimes known as 'dog-whistle politics': "Just as only canines can hear a dog whistle, only prejudiced voters are aware of the racist connotations of a politician’s statement, according to the theory."

The Post contacted the Republican National Committee and McCain's Senate office for comment on the matter, neither of whom immediately responded.