That short time frame winnowed down the field. Among those deciding to sit it out are City Attorney Dennis Herrera, state Assemblyman David Chiu and San Francisco Assessor Carmen Chu. All three had been toying with the idea of a run, but ultimately decided not to jump in.
Breed, as board president, became acting mayor upon Lee's death Dec. 12. But whether she stays in the mayor's office in that role until the election is still in doubt. On Tuesday, Breed accepted a request by Supervisor Aaron Peskin to schedule a vote for next week on who should serve as interim mayor in the coming months.
If no candidate secures the necessary six votes -- and that appears unlikely right now -- Breed will remain in the job with the ability to raise her profile and campaign cash, make appointments and shape the next city budget.
Progressive groups, still stung by the way moderate Ed Lee used his appointment as acting mayor in 2011 to win a full term, are hoping to at least blunt Breed's advantage of sudden incumbency by replacing her with an interim mayor. Some potential candidates who decided not to run may have calculated they could not overcome Breed's advantages.
The race could turn on whether voters want to stay the course charted by Mayor Lee or go in a different direction, and which candidate is best positioned to do that.
A recent poll found the race starting off with two clear leaders. The survey of 627 registered San Francisco voters found Mark Leno with 26 percent support and London Breed 20 percent.
The June election will ultimately be decided by ranked-choice voting, meaning candidates will have to vie to be voters' second and third choice.