After decades of sitting on the sidelines of politics, Bill Gates, one of the richest people in the world, has said privately that he recently donated about $50 million to a nonprofit organization that is supporting Vice President Kamala Harris’s presidential run, according to three people briefed on the matter. The donation was meant to stay under wraps. Mr. Gates, one of the founders of Microsoft, has not publicly endorsed Ms. Harris, and his donation would represent a significant change in the strategy that has previously kept him away from gifts like this. In private calls this year to friends...