Cory Booker
Naturally, because United is written from Booker's point-of-view, he is the main character in the book. In the book, in fact, Booker discusses his life from his birth in northern New Jersey to two executives at IBM to his election to the office of Mayor of Newark, New Jersey to, finally, his career as a United States Senator from New Jersey.
As readers go through the book, they come to learn that Booker is an intelligent, ambitious, tenacious, and driven individual who cares deeply for the common man - from the snowplower to the police officer to the bartender.
Barack Obama
Not only was Barack Obama the first African-American President of the United States, but he was a key influence on Booker's life and on his political career. After first being elected as President in 2008 (he was a Senator from Illinois before that), Obama's was again elected in 2012. Although parts of his Presidency was controversial, Obama was a mostly well-liked President who was noted for his calm demeanor, intelligence, and his tremendous public speaking skills.
When Booker went on to run for President in 2020, he looked to Obama's 2008 campaign and tried to update his platform and his message to galvanize the potential voters like Obama had in 2008.
