Reports suggest Biden hopes to prove he’s up to the job, starting with a Friday sit-down interview on ABC News with George Stephanopoulos. Maybe he will succeed. Maybe he won’t. Either way, conversation has already started about what happens if Biden steps aside. And that has put the focus on Vice President Kamala Harris.

Harris is the most likely to take Biden’s place in almost any scenario, whether she receives some kind of formal endorsement from Biden and party leaders or simply has to prevail in an open contest for convention delegates. Among the plausible contenders, she has the most name recognition and stature. She, and only she, can say she’s been in the room and on the world stage with Biden, and performed under White House-level scrutiny.