Late one night in December, 2009, a black Chevy Tahoe in a caravan of cops and residents moved slowly through some of the most dangerous neighborhoods of Newark. In the back sat the Democratic mayor, Cory Booker, and the Republican governor-elect of New Jersey, Chris Christie. They had become friendly almost a decade earlier, during Christie’s years as United States Attorney in Newark, and Booker had invited him to join one of his periodic patrols of the city’s busiest drug corridors.