A recent survey from Morning Consult echoes this. Urban and young consumers were more likely to say they would react favorably to a company that advocated the right of protesters to kneel during the national anthem. Indeed, on social media — where the nation’s youth live and breathe — Mr. Kaepernick attracted more than one million responses on Instagram, Facebook and Twitter in the hours after he announced the Nike partnership, according to ListenFirst, a social media analytics company.

On Twitter, more than 100,000 posts in the first 24 hours included the hashtag #BoycottNike, but the firm’s analysis showed “more support for Nike and Kaepernick than we see negative actions taken against them,” said Jason Klein, co-chief executive of ListenFirst. The campaign, which was announced with a simple tweet by Mr. Kaepernick, has also generated at least $43 million in free advertising for Nike, according to one estimate.

The Kaepernick partnership comes on the heels of a report by The New York Times that Nike was a hostile and abusive work environment for women. In August, two women filed a class-action lawsuit against Nike claiming gender-pay discrimination and sexual harassment.

Still, the timing of the partnership with Mr. Kaepernick caught many industry veterans, as well as the N.F.L., off guard. For more than a year, Nike has virtually ignored Mr. Kaepernick and declined to use him in any of its marketing campaigns, even though he has been under contract to the Oregon-based company since 2011.

The new partnership comes months after Nike extended its agreement with the N.F.L. to provide on-field uniforms for all 32 of the league’s teams. On Monday, when asked if Nike had informed the N.F.L. about the campaign beforehand, a Nike spokeswoman said that “Colin is not currently employed by an N.F.L. team and has no contractual obligation to the N.F.L.”

The N.F.L. did not address the campaign Monday, then released a statement Tuesday in which it said, “The social justice issues that Colin and other professional athletes have raised deserve our attention and action.”