A month after John Kerry’s narrow loss to George W. Bush in 2004’s Presidential election, Eli Pariser boasted to his MoveOn associates, “Now it’s our party. We bought it. We own it.” The huge question raised by this audacious declaration was, “Who are ‘we’?” That query is answered in David Horowitz and Richard Poe’s new book, The Shadow Party—a work that also explains how this political takeover was accomplished. In the authors’ own words, “This book documents how, through an extraordinary series of political, legal, and financial maneuvers, an unlikely network of radical activists and activist billionaires gained de facto...