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...