Farrell is seeking to amend a San Francisco law that governs public financing of candidates running for office in the city after the U.S. Supreme Court struck down a similar law in Arizona last month.
Under San Francisco's existing law, passed by voters as Proposition O in 2000, mayoral candidates are each eligible to receive $900,000 in public matching funds, while supervisorial candidates can receive $89,000.
The city's Ethics Commission is allowed to increase those figures for candidates if there is a certain level of private money being spent against a candidate by opponents.
The new legislation would eliminate the additional public matching funds, the provision that was struck down in Arizona's case.
There are more than 30 candidates running for mayor, and many of the leading candidates are receiving public financing.
Farrell said the city can either change its law "or wait for a plaintiff to sue ... and I don't want to waste additional taxpayer funds" in a legal battle.
To make a change to the law, the Ethics Commission has to approve the change by a four-fifths vote, which it did on Monday.
The Board of Supervisors will have to approve the change by a two-thirds vote.