Nike shares fell Tuesday after the reveal of a new ad campaign for the 30th anniversary of "Just Do It," featuring former San Francisco 49ers quarterback Colin Kaepernick, pulled a wave of responses, both for and against the marketing materials, on social media.

The stock closed the day down roughly 3.2 percent, as #NikeBoycott was trending on Twitter.

On Monday, Kaepernick shared a photo of the campaign on his Twitter account. He's been a polarizing figure in the sports universe ever since he decided not to stand for the national anthem during a 2016 NFL preseason football game to protest racial injustice.

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Kaepernick has been a Nike-sponsored athlete since 2011, a source familiar with the deal told CNBC. Until this week, it was unclear how Nike would work again with the athlete. Kaepernick had previously been featured in several campaigns for Nike football and training. However, he's not currently signed with a National Football League team.

The NFL didn't respond specifically to Nike's decision to use Kaepernick in their ad campaign but issued a statement Tuesday afternoon saying: "The social justice issues that Colin and other professional athletes have raised deserve our attention and action. ... We embrace the role and responsibility of everyone involved with this game to promote meaningful, positive change in our communities."

Nike did see backlash on social media, though there are also plenty of people backing the campaign. Other athletes including Serena Williams, LeBron James, Lacey Baker, Shaquem Griffin and Odell Beckham Jr. are also being featured in the ads, according to Nike. But Kaepernick has taken the spotlight.

The idea of the campaign is to showcase athletes overcoming obstacles, a source familiar with the ads said. A photo of Williams playing tennis says, for example: "Girls from Compton don't play tennis. They own it." Another one of Beckham Jr. playing football reads: "Don't wait until you've won a ring to play like it."