Because it has been so effective in the past
"San Francisco bashing is about to intensify. Three Democratic politicians with strong ties to San Francisco - Jerry Brown, Gavin Newsom and Kamala Harris - are engaged in tough statewide election battles and political experts say the GOP will probably seize on the city's reputation as a bastion of left-wing politics to attack them."
Gubernatorial candidate Brown was born in San Francisco, served as Oakland's mayor and lives in the East Bay city while serving as the state attorney general; Newsom, vying for lieutenant governor, was born in San Francisco and is the city's mayor; and Attorney General candidate Harris was born in Oakland and is the San Francisco district attorney."
Not only are they determined to demonize the 2nd largest metropolitan area in the state. Immigrants are the target as well.
"That won't stop Republican candidates from trying, however. Some of the rhetoric has already begun: Meg Whitman, the Republican nominee for governor, for weeks has derided the "so-called sanctuary cities like San Francisco who thumb their nose at our laws," in television ads. That sanctuary city policy bars city employees from assisting federal immigration officials in detaining illegal immigrants unless the person is accused of a crime."
http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2010/06/13/MNHQ1DSIS6.DTL