Newark, New Jersey (CNN) Sen. Cory Booker marked the start of a new phase in his campaign for president Saturday with his first major rally, set in the heart of the city where he got his start in politics as a councilman and then mayor.

Billed as his "hometown kickoff," the event afforded Booker an opportunity to turn a page after a muted start to his bid in which the New Jersey Democrat has lagged in polling and fundraising behind half a dozen rivals.

From a stage in Newark's Military Park, Booker imbued his core theme of "justice for all" with a fresh sense of urgency — invoking Dr. Martin Luther King Jr.'s "Letter from a Birmingham Jail," written at this time in 1963, and its declaration that "justice too long delayed is justice denied."

"We are here today to say, we can't wait," Booker said.

Booker also reiterated his commitment to running a positive campaign as President Donald Trump and others seek to "make us fear each other, dislike each other, and hate each other."

Read More