Colin Kaepernick, who kick-started a nationwide debate about police brutality and athletes’ rights to protest, is accusing the NFL and its 32 teams of blocking him from getting a job.

The quarterback’s case contains “considerable” challenges, a former NLRB chairman told Bloomberg Law.

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“The challenge for Kaepernick is very considerable,” William Gould told Bloomberg Law. Gould is a former chairman of the National Labor Relations Board and now a labor professor at Stanford Law School.

Kaepernick’s attorney must prove teams collectively agreed to not sign Kaepernick despite his superior qualifications, Gould said. Whether Kaepernick is a superior quarterback may be easier to prove than whether the teams agreed not to sign him.

“The quarterback position is more statistically driven and there is more objective evidence that they can produce,” Gould said. “It seems to me there are more objective ways to measure effectiveness in that position than for a lot of others.”

But proving teams met behind the scenes to freeze Kaepernick out of the league will be harder. Finding an email or an instance of a meeting where team owners agreed to not hire Kaepernick would be a “smoking gun,” Gould said. “We’ll find out, but it’s unlikely that Kaepernick will be able to show that kind of thing.”