Conversation so far:
What are Joe's characteristics as senator?
characterized Biden as "self-confident" and "compulsively ambitious".
Was Joe likable as senator?
"Biden is the kind of fundamentally happy person who can be as generous toward others as he is to himself..
Does Joe have a lot of friends in politics?
CANNOTANSWER.

Question: When did he become senator?
Self-contained rewrite: When did Joe Biden become senator?