New York City mayor Bill de Blasio on Thursday announced officially that he is joining the Democratic presidential primary to challenge Trump in 2020.

"I'm a New Yorker, I've known Trump's a bully for a long time. This is not news to me or anyone else here. And I know how to take him on," de Blasio said in a video posted online. "Don't back down in the face of a bully -- confront him, take him on."

He added: "Donald Trump must be stopped -- I've beaten him before and I will do it again. I'm Bill de Blasio, and I'm running for president because it's time we put working people first."

De Blasio's entrance into the race will bring the total number of candidates seeking to challenge President Donald Trump to 23, with nearly nine months before the first votes are cast.