Jerry Brown's toughest opponent -- himself?

California Gov. Edmund G. (Jerry) Brown Jr. is battling two tough opponents in his attempt to become a US senator: Republican opponent Pete Wilson - and himself.

A just-published California Poll shows Democrat Brown trailing San Diego Mayor Wilson by 11 points (37 percent to 48 percent, with 15 percent for others or undecided). But Mervin D. Field's respected opinion sampling institute also found that 43 percent of those polled in August supported Wilson because they ''don't like Brown'' or perceive Wilson as the ''lesser of two evils.''

Governor Brown, aware that his negative image in the eyes of many Californians is his major handicap, spent some $800,000 for a series of television commercials last month urging voters to ''take another look at Jerry Brown.'' If they did, the Field Poll indicates, they apparently did not see a ''new Brown.'' Asked why they did not support him, 34 percent of those sampled in the August poll said he ''changes his mind too much'' and ''flip-flops on issues,'' and 18 percent said Brown ''has done a poor job as governor.''

The poll shows just 57 percent of Democrats supporting Brown, 27 percent for Wilson, and 16 percent ''undecided/others.'' Wilson has solid GOP voter support - 77 percent.