Kennedy split the split the Democratic Party, causing people not to bother to vote. Plus it gave Reagan many talking points, which then started the move of Union People to the GOP.
"a few days before the announcement went badly, however. Kennedy gave an "incoherent and repetitive"[9] answer to the question of why he was running, and the polls, which showed him leading Carter by 58–25 in August now had him ahead 49–39."
Who knows how this could have gone had that interview not bombed his ratings.
