In San Francisco, the middle of the road has its hazards

It's a clear case of political double jeopardy. San Francisco Mayor Dianne Feinstein, who was getting set to seek her second full term next November, suddenly faces a recall election April 26. So the city has to pay for two elections and the mayor for two campaigns in less than seven months.

It is estimated that the recall election will cost the city $450,000; Mayor Feinstein expects to spend some $350,000 defending her record in office.

This situation, much lamented by the local news media as well as most public officials, resulted from Mayor Feinstein's successful drive last fall to enact a San Francisco handgun ban. The ordinance, recently struck down by the California Supreme Court, angered a small, militant group here which calls itself the White Panthers. The avowedly communist Panthers are vociferous supporters of the ''constitutional right'' to bear arms.

The group had little difficulty collecting 35,000 signatures on a recall. Only 19,780 valid signatures were needed under the city charter's provision that a recall election can be initiated by 10 percent of the vote in the previous election; more than 20,000 of the signatures collected were validated.

Aside from the nuisance and expense, it would appear the mayor has little to fear. Since assuming the office in 1978, when Mayor George Moscone was assassinated, and then winning a four-year term in 1979, she has compiled a scandal-free, tight-ship record. For a city of its size (just under 600,000) San Francisco's problems are not large scale, and the treasury even holds a modest surplus.

Mrs. Feinstein (who has publicly kept the name of her late husband to avoid identity problems, although she remarried in 1979) demonstrated her self-control in the aftermath of the Moscone tragedy and has maintained an evenhanded image since.

But it's her very evenhandedness, her generally middle-of-the-road stance in a city of many extremes, that could make this recall election dangerous for her. The 35,000 signatures collected by the White Panthers were from people in every sector of the city - representing what one commentator called ''a smorgasbord of dissatisfaction.''

Thus, Mayor Feinstein's strong suit in the past - her appeal to moderates - could be a weakness. There are no other candidates to run against, no ''race'' to attract voter attention. If her critics go to the polls while others complacently ignore the election, the mayor could see her reelection bid derailed.

If she decisively defeats the recall, Mrs. Feinstein could emerge with such strength that prospective opponents may see little point in challenging her in November.