Sen. Cory Booker (D-N.J.) announced Monday that he is dropping out of the 2020 presidential race.

The big picture: Although Booker was seen as a rising star in Democratic politics, he struggled to shine in the 2020 race. In recent weeks, he focused his ire on the dwindling diversity in the Democratic field, saying in December that the race had "more billionaires than black people" after Kamala Harris' departure.

He also went head-to-head with frontrunner former Vice President Joe Biden in response to his comments on segregationist senators, and Biden's tendency to invoke President Obama as a political play.

Booker also confronted Biden over his continued opposition to legalizing recreational marijuana. He argued "marijuana in our country is already legal for privileged people" at the November debate.

Having failed to qualify for the December debate, Booker circulated a letter that month asking the Democratic National Committee to ease future debate standards. All seven candidates who did qualify for the December debate signed on.

Yes, but: Booker's past had vulnerabilities. In the 2014 election cycle, he was the top recipient of Wall Street money, and he's also seen as a friend of Silicon Valley and the the pharmaceutical industry.

The state of play, via Axios' Alexi McCammond: His decision comes just one day before the next Democratic debate, days before the start of President Trump's impeachment trial (where he'll serve as a juror) and three weeks before the Iowa caucuses.

What he's saying:

"I will be doing everything in my power to elect the eventual Democratic nominee for president, whomever that may be, and to elect great Democrats to the Senate and up and down the ballot. 2020 is the most important election of our lifetimes — we have to beat Donald Trump ... but beating Trump is the floor, not the ceiling."

— Cory Booker, via a campaign email to supporters

Watch his video:

What's next: Booker is up for re-election to his Senate seat in 2020.

One bright spot: a SurveyMonkey poll for Axios showed him as a potential presidential choice for 24% of Democrats in 2024 — behind only Pete Buttigieg and Kamala Harris.

Go deeper: 2020 presidential election: Track which candidates are running