Supervisor Tom Ammiano proposed yesterday that San Francisco scrap its system of costly and often- brutal runoff elections and replace it with instant runoffs.

His idea is for voters to pick their top candidate for an office, along with their second and third choices. When all the votes are counted, the last-place candidate in the race would be dropped, if no candidate got a majority.

That losing candidate's votes would then be transferred to the voters' second choice. Voters whose second choice was eliminated would have their ballots shifted to their third pick.

This process would continue until one candidate got a majority, hence the term instant runoff.

Currently, if no candidate wins a majority in the general election, the top two finishers advance to a runoff six weeks later. Five of the past six mayoral elections have been decided in runoff elections.

Ammiano needs the support of a majority of the 11 supervisors to put his proposed City Charter amendment before voters on the March 2000 presidential primary ballot. He wants the new method of electing all citywide officials and supervisors to take effect with the November 2000 ballot, when the supervisors will be elected in districts for the first time in more than 20 years.

``This system will avoid the potential for costly runoffs, which can cost up to $1 million to administer,'' Ammiano said in proposing his system at yesterday's weekly board meeting. ``It would also discourage negative campaigning, encourage coalition- building and reduce fund-raising demands.''

If Ammiano's proposal gets on the ballot, it would not be the first time San Francisco voters have been asked to change their system of voting in local elections. In November 1996, they rejected Proposition H, a Charter amendment that called for a complicated system of preference voting in citywide supervisors elections.

That measure had the support of a broad coalition that included a majority of the supervisors, the county Democratic Party and organized labor, but voters apparently found the proposed system too tangled.

Ammiano's proposal is identical to one that Santa Clara County voters decided to adopt in November for their countywide elections.

Opposition to the idea immediately surfaced from Adam Sparks, who worked to defeat the 1996 plan.

He said the current system remains the best because it gives voters ``maximum control and maximum flexibility.''

Sparks also said voters' choices about which candidate they will back often changes before a runoff, so the second round of campaigning is valuable for voters.

``Your decision could also be determined by who your candidate is running against,'' Sparks said. ``A lot of times your choice for second could change, depending on who is in the runoff.''

Steve Hill of the Center for Voting and Democracy, which helped draft the proposal, said the instant runoff would promote civility in campaigns.

``In places that use it, candidates have to be careful what they say about their opponents because they want people to vote for them as their second choice,'' Hill said.