Washington: A new study shows that negative advertisements targeting US President Barack Obama in 2008 depicted him with very dark skin, and that these images would have appealed to some viewers' racial biases.

The finding reinforces charges that some Republican politicians seek to win votes by implying support for racist views and ethnic hierarchies, without voicing those prejudices explicitly. The purported tactic is often called "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.

Research has found that Barack Obama's skin appears to have been darkened in certain Republican attack advertisements screened during the 2008 campaign. Credit:Washington Post

That debate has been prominent in the 2016 campaign, primarily targeting Donald Trump, but it has existed in almost every recent presidential election. To hear their opponents tell it, when Republican politicians say they oppose a generous welfare system, they really mean black beneficiaries are lazy. If they endorse strict immigration enforcement, they really mean that Latinos are criminals, critics say.