Former San Francisco mayor Dianne Feinstein, who lost a spirited battle for the governorship of California to Republican Pete Wilson, is planning to seek Wilson's former Senate seat in 1992, when California will elect two senators.

Feinstein surprised many of her fellow Democrats by declaring Sunday that she is "exploring" a run for the seat "if I can avoid the possibility of primary opposition." The seat is held by Republican John Seymour, a relatively unknown former state legislator from Orange County whom Wilson named to the Senate earlier this month.

If Feinstein wins, she would face reelection in 1994, when the term expires. Many had expected Feinstein to run for the full six-year Senate term of Democrat Alan Cranston, who is retiring in 1992.

But a Democrat with close ties to Feinstein said she had made a "calculated decision" to run for the two-year term because she alone among the Democrats could raise the enormous sums needed for back-to-back Senate races in two years. Wilson spent nearly $13 million to win reelection to the Senate in 1988.

Feinstein's husband, Richard Blum, is a wealthy financier who seeded her gubernatorial campaign fund by advancing personal loans of about $3 million to the bid, which cost at least $15 million. In addition, she developed an impressive list of contributors in that race and is believed to have the best name recognition of any Democrat in California.

As many as seven Democrats, probably including former governor Edmund G. "Jerry" Brown Jr., are expected to vie for the nomination to the Cranston seat. The list of Republican competitors for this seat also is likely to be long and include Ed Zschau, who lost narrowly to Cranston in 1986.