Warren Is Preparing for 2020. So Are Biden, Booker, Harris and Sanders.

Elizabeth Warren, 69, now leads a small advance guard of Democrats who appear to be moving deliberately toward challenging President Trump. Former Vice President Joseph R. Biden Jr., wielding a political network cultivated over decades, has been reasserting himself as a party leader, while Senators Cory Booker of New Jersey and Kamala Harris of California have emerged as fresher-faced messengers for the midterms. And Senator Bernie Sanders of Vermont, the runner-up in the 2016 primaries, has been acting like a candidate as he considers another race. All five have been traveling the country, raising money for Democrats and gauging the appeal of their personalities and favorite themes.