WASHINGTON (AP) - U.S. Sen. Cory Booker on Friday declared his bid for the presidency in 2020 with a sweeping call to unite a deeply polarized nation around a "common purpose."

The New Jersey Democrat, who is the second black candidate in a primary field that's already historically diverse, delivered his message of unity amid an era marked by bitter political division.

"I believe that we can build a country where no one is forgotten, no one is left behind; where parents can put food on the table; where there are good-paying jobs with good benefits in every neighborhood; where our criminal justice system keeps us safe, instead of shuffling more children into cages and coffins; where we see the faces of our leaders on television and feel pride, not shame," Booker said in the video, subtly jabbing at President Donald Trump.

"It is not a matter of can we, it's a matter of do we have the collective will, the American will?" he added. "I believe we do."

Booker enters what's shaping up to be a crowded presidential primary, with three of his fellow Democratic senators - Elizabeth Warren of Massachusetts, Kamala Harris of California and Kirsten Gillibrand of New York - already either declared or exploring a run. But he's spent months telegraphing his intentions to join the race, visiting the early-voting states of Iowa, New Hampshire and South Carolina to build connections with key powerbrokers.

Booker, a former mayor of Newark, New Jersey's largest city, won a special Senate election in 2013 to replace Democrat Frank Lautenberg and then won a full Senate term in 2014. He will be able to run for a second full Senate term in 2020 while running for president, thanks to a law that New Jersey's governor signed in November.

But that doesn't mean the 49-year-old's path to the nomination will be easy. As many as five more Democratic senators could soon mount their own primary bids, creating a competition for voters' attention, and several of Booker's rival presidential hopefuls bring higher name recognition to a race that may also feature popular former Vice President Joe Biden. Booker also will likely stand alone as an unmarried candidate, though he brings a compelling personal biography that could help elevate his message of bringing Americans together around what he described as "common purpose."

Booker's father grew up in a low-income community in North Carolina, and the senator has recalled his family's later struggle to settle in suburban New Jersey amid discrimination against black homebuyers. The senator has brought a heartfelt and passionate style to his achievements in the Senate, at times fusing his personal spirituality with policy proposals that focus on social justice. Booker played a key role in the bipartisan criminal justice reform bill that Trump supported last year, for example, a deal he helped strike two months after sparring with Republicans during the battle over Supreme Court Justice Brett Kavanaugh's confirmation.

In his announcement video, Booker invoked the fight against slavery and the role of immigration in building the nation's character.

"The history of our nation is defined by collective action; by interwoven destinies of slaves and abolitionists; of those born here and those who chose America as home; of those who took up arms to defend our country and those who linked arms to challenge and change it," he said.

Born in the nation's capital but raised in New Jersey, Booker made a name for himself as Newark mayor by personally shoveling the snow of residents. He has $4.1 million left in his campaign coffers that could also be used to assist his presidential run. Rather than opening an exploratory committee to test the waters, Booker took the direct step to open a campaign seeking the Democratic nomination.