Eleven Boston Mayoral candidates participated in a debate at the Boston Teachers Union (BTU) Hall Wednesday night with hopes of earning the endorsement of Boston’s largest union.

Dan Conley was the only remaining candidate in the race not to attend, citing another commitment, according to the Boston Globe. Conley has advocated for the cap on charter schools to be lifted and made it a central issue in his campaign.

David Bernstein, a contributing editor for Boston magazine served as moderator of the BTU debate—focusing predominantly on the “toxic” issue of charter schools—as teacher union members and school administrators filled the Hall, according to the Boston Globe.

Candidates Martin Walsh, Bill Walczak, John Connolly and John Barros “raised their hands with little hesitation” when Bernstein asked if charter schools were having a positive effect on Boston’s school system. The candidates’ approval reportedly drew murmurs from the crowd—many of whom belong to the BTU, which opposes more charter schools.

The BTU is contemplating endorsing their first mayoral candidate since 1991. Former BTU president Edward Doherty was the last candidate to receive the union’s endorsement during his unsuccessful run against Mayor Raymond Flynn.

Representing 5,500 teachers and other school workers, the BTU could provide any one the the 12 candidates with a massive voting boost, in what promises to be a tight election.

Based on “color-printed scoreboards”—distributed by the union before the debate—four candidates emerged as potential favorites of the BTU: Felix Arroyo, Rob Consalvo, Mike Ross and Charles Yancey.

John Connolly appeared to fall out of contention for the BTU endorsement.

Candidates scorecards included information about which candidates sought endorsement from pro-charter school union, Stand for Children, and who voted in favor of the teacher union’s contract last year, according to the Boston Globe.

Connolly was endorsed by Stand for Children and voted against the contract.

Boston Teachers Union President Richard Stutman told the Globe that the union won’t have a decision until after the primary election on Sept. 24, which will trim the field to two candidates.