He ran in 1988 in his mid-40s as a champion for a new generation, then dropped out in humiliating fashion over allegations that he plagiarized a speech. He tried again in 2008 as an experienced insider, only to watch in frustration as all the attention focused on the party’s two biggest celebrities, Sens. Hillary Clinton and Barack Obama. As the outgoing vice president in 2016, Biden pondered a run but bowed out as establishment support coalesced around Clinton.