Steve Jones wrote an excellent article this week describing the potential challengers for the Mayor’s office:

Mayor Gavin Newsom has long been considered a lock for reelection next year, a belief driven by his same-sex marriage gesture, hoarding of political capital, personal charm, and high approval ratings. Yet Guardian interviews with more than 20 political experts and insiders from across the ideological spectrum indicate that Newsom may now be more vulnerable than ever.
Just as San Francisco politicians are starting to calculate whether to run, the Newsom administration has suffered a series of political setbacks.

Randy Shaw also weighed in and described, quite accurately, some of the problems of the 1999 race against Willie Brown, meaning that Brown won re-election solely with the support of the right and felt no need to engage the left outside of organized labor:

While Brown faced progressive attacks, the more conservative side of his 1995 electoral coalition fought hard for the mayor and earned his political loyalty for the next four years. Brown interpreted the 1999 campaign as showing that progressives could not be trusted, and decided to stop trying to meet their needs.

Brown became a much more pro-development mayor after the 1999 runoff, and returned to his historic position as a strong ally of landlords. He continued to back organized labor, which strongly supported him despite Ammiano’s long history of union advocacy (Local 21 was the only major union to endorse Ammiano).

A progressive candidate challenging Newsom in 2007 would have to avoid both of the Hayden and Ammiano scenarios. This means that the candidacy has to be backed with real grassroots support, and it cannot run a scorched earth campaign that portrays Newsom and his supporters as evil.

That said, I do believe that the wins in the Supervisorial race were largely built on the work we did in the Mayor’s race in 1999.