The survey comes after multiple unsuccessful attempts by the council to improve voter turnout.
The final results of the election will be announced Wednesday.
The preliminary votes are from people who voted in-person Sunday.
Approaching the end of his fifth term and a November election, council member Dennis found that now is the right time to leave his seat open.
The act comes ahead of November city elections and will last into the 2024 presidential race.
Brennan has served on the council since 2014.
Turnout dropped 2 percentage points since that election.