The move paved the way for Democrats to choose a new nominee to challenge Senator Susan Collins in a race both parties see as key to the control of the Senate.
With plans still being finalized, officials are discussing a convention some time during the weekend of July 25-26 and are weighing options for choosing delegates.
The state Democratic Party has said it will pick a replacement through a nominating convention before a July 27 deadline. Candidates are already lining up.
