Lewis brings up Eisman’s Republican past to show that, while politics may not always play an obvious role in the story, they are often part of the subtext, particularly for Eisman. His experiences on Wall Street completely change his worldview, and this is what leads him to crusade against what he sees as terrible practices in the industry. He didn’t start as an ideologue; instead, seeing what was really going on radicalized him. His socialist politics and his difficulty raising money help establish that, despite his privilege and experience, he’s still something of an outsider to the industry.