The New Jersey senator's speech is not a filibuster, as he is not delaying the passage of any bills or blocking the confirmation of any Trump nominees.
His stance is taking place after nine Democrats, including Senate Minority Leader Chuck Schumer, D-N.Y., and Sen. Angus King, I-Maine, voted with nearly all Republicans to advance a GOP spending bill last month.
The votes prevented a government shutdown but angered liberal voters who want their lawmakers to stand up to Trump, which Booker acknowledged.
"I've been hearing from people all over the state, and indeed all over the nation, calling upon folks in Congress to do more, to do things that recognize the urgency, the crisis of the moment," Booker commented in a post on social media.