Study finds attack ads from the 2008 presidential campaign appeared to show Obama with darker-than-usual skin - these are the ads

A study by Stanford University researchers that analyzed more than 100 ads from the 2008 US presidential election has found that Sen. John McCain's (R-Arizona) campaign used images of then-Senator Barack Obama (D-Illinois) that made his skin appear darker than usual.

The study, published in the academic journal Public Opinion Quarterly in December, suggests that the images could have been intended to tap into possible racial biases of some viewers.

"Ads that portray black candidates with a darker complexion might prime negative stereotypes about blacks, damaging the candidate's election prospects in a way that has nothing to do with political fitness for office and is difficult to detect," the researchers wrote in the study.

The study's relevance, more than eight years after Obama was first elected to the White House, comes as the 2016 US presidential primary race heats up.

In the GOP field, where real-estate mogul and current front-runner, Donald Trump has consistently led in the polls, the candidate's controversial approach to matters of immigration, homeland security, and civil rights has relied on similar methods for attacking minority groups.

Trump's description of some Mexican immigrants as "rapists" and "drug dealers" and his suggestions that Muslims should be temporarily banned from the US have effectively triggered similar, negative biases against people of color.

In 2008, the depictions of Obama's skin tone varied throughout the campaign season, but he appeared especially dark-skinned in Republican attack ads that aired closer to election day, according to the Stanford study.

Meanwhile, McCain's skin appeared gradually lighter over time in the same ads.

The disparity in skin tone is not necessarily a result of deliberate manipulation by the McCain campaign. The researchers noted that "more shadowed" images of Obama may have been selected to match the sinister tone of the attack ads.

The darker-looking images could have also been a result of video compression in the uploading process, according to a Factcheck.org analysis of an anti-Obama ad produced by Hillary Clinton's 2008 presidential campaign.

According to the study, Obama's skin appeared the darkest in ads connecting him to crime, including ones that attempted to link the candidate to Bill Ayers, the former leader of the radical left-wing organization Weather Underground:

Another ad sought to tie Obama to alleged misconduct committed by the Association of Community Organizations for Reform Now, known by the acronym ACORN:

Other GOP attack ads in question paint Obama as disrespectful ...

... and irresponsible ...

In another portion of the study, researchers showed participants images that depicted Obama with a lighter or darker-than-usual skin tone. The images were manipulated accordingly for the purposes of the study.

The researchers found participants were quicker to think of negative black stereotypes - such as laziness and criminal behavior, for example - after seeing the darker-skinned version.

The findings give new ammunition to critics who claim the Republican Party may use implicit racial appeals to attract some voters.

"When you blacken somebody's skin, you're doing it for only one reason. It's to make them look darker and more sinister," First Amendment attorney Daniel Barr told Arizona's KTVK-3 TV.

The tactic may not be limited to the GOP, either. The study also references an attack ad from Hillary Clinton's 2008 Democratic primary campaign that allegedly depicted Obama with darker and wider-than-usual facial features.

You can view hundreds of campaign advertisements from both parties at Stanford University's Political Communication Lab.