The revelations of what Platner was and is were weekly news since or before April. The national Dem Party knew almost everything, including at least one credible rape accusation, before Maine’s primary. Platner was not the only Dem candidate, the former governor of Maine was also running. The Dem Party just wanted to to support the DSA’s candidate.
Somewhat more to the point, Maine voters knew. The credible rape accusation was reported in the NYT several days before Primary day. Whatever the timing of individual voters’ votes, every voter knew at least some of the problems that should have sunk his candidacy.