San Francisco’s public financing program rules could change in time to impact the November mayor’s race.

After a U.S. Supreme Court decision ruled against a public financing program in Arizona, San Francisco’s program appears vulnerable to a legal challenge.

Supervisor Mark Farrell introduced legislation Tuesday that would tweak the rules, which he said The City must do to avoid a legal challenge.

“Today, we have a public campaign financing law on our books which is in conflict with the U.S. Supreme Court,” Farrell said. “I don’t want to waste additional taxpayer funds” in fighting a legal challenge.

The leading candidates in the mayoral race are participating in the public financing program. Doing so, they agree to spend no more than $1,475,000 on their campaign in a total combination of matching public and private contributions. The most public funds a candidate can receive is $900,000. But if independent expenditures for or against a candidate exceed that limit, then the spending limit is lifted accordingly and the candidate can receive more matching public funds.

Under Farrell’s legislation, a candidate would not be able to receive any additional public funds beyond the cap if the spending is limit is increased. The candidate could, however, raise more private contributions.

Advocates of public financing worry the rule change would hamper candidates’ ability to respond to aggressive and well-funded attacks. And they also question whether San Francisco’s program differs enough from Arizona’s to be able to withstand a legal challenge.

It would take eight votes by the Board of Supervisors to approve the legislation.

jsabatini@sfexaminer.com