Gov. Edmund G. (Jerry) Brown Jr., who two years ago defeated Jimmy Carter in five straight presidential primaries, now finds himself locked in a surprisingly tight reelection battle against an opponent who has been described as being "as exciting as a mashed potato sandwich."

A survey by Mervin Field's well-regarded California Poll earlier this month put Brown ahead of two-term Attorney General Evelle J. Younger by only five percentage points with 11 percent of the voters undecided. Furthermore, Field's survey found that significant numbers of voters considered Brown to be "wishy-washy" or "opportunistic" and also "inconsistent on Proposition 13."

Brown's slip in public approval seems even more significant in light of the upset defeat Tuesday of Massachusetts Gov. Michael S. Dukakis, whose spartan personal style and liberal views often have been compared to those of the California governor. And Younger is trying to use the same themes of higher state spending and taxation that brought Dukakis down.

While Brown is still favored to win, some of his own supporters now acknowledge privately that the election has yet to be decided. Recently, a key strategist said that Field's findings were accurate and predicted a close outcome in November.

"Jerry has a lot of negatives," this strategist said."Some people view him as inconsistent on Proposition 13. Others are bothered by his opposition to capital punishment.Above all he's an incumbent in a time when it's not popular to be an incumbent."

Nonetheless, the results of Field's survey came as a surprise to many politicians, including some Republican activists who had just about given up on Younger's chances.

Brown took the political initiative immediately after the June primary, which was marked by the landslide passage of the tax-cutting Proposition 13. Though he had denounced the measure as a "rip-off" and a "consumer fraud" until a few days before the election the governor won the plaudits of even his critics by the speedy way he moved to implement the initiative.

While this was going on Younger underwent surgery for removal of a kidney stone took a recuperative vacation in Hawaii and launched a series of radio commercials which attacked Brown's "flip-flops" on various issues.

Many politicians thought these ads were overshadowed by Brown commercials that were both more forceful and more personal. The Younger campaign seemed so quiescent and defensive that one Republican political consultant, former Gerald Ford strategist Stu Spencer, publicly declared that Brown was wrapping up the election while Younger was waiting for the campaign to begin.

The reason that the race is a close one probably has less to do with Younger than the increasing voter skepticism about the governor himself.

After four years of unconventional governorship, Brown has accumulated some very conventional voter grievances that have tended to stamp him as a "politician" at a time when politicians are unpopular.

Field's survey, for instance, found that 47 percent of the voters - including nearly a third of those who say they planned to vote for Brown - find him inconsistent on Proposition 13. This is the highest negative mention given either candidate in the survey.

Forty-four percent of the voters find Brown "opportunistic or seeking higher office" while 41 percent label him as "wishy-washy" or say he "takes both sides on the issue."

One the other hand, two-thirds of those surveyed find Brown "intelligent" and 61 percent say he is "hard working." He also wins credit for sympathy to minorities and the disadvantaged and for having "high ideals." Younger does well in all of these categories but not as well as Brown.

But the negatives of Younger also are less defined, causing Field to conclude that feelings about the attorney general are less polarized. The highest negative mention about Younger (30 percent) is that he has "accumulated too much in pensions at public expense" - a favorite theme of the Brown commercial.

The question now is whether Younger, described as "dull" even by his own campaign strategists, can take advantage of Brown's perceived weakness.

The attorney general is a plodding speaker - leading defeated GOP rival Ed Davis to compare him to a mashed potato sandwich. Davis, a former Los Angeles police chief who is something of a specialist in unappetizing culinary metaphors, also remarked that trying to pin Brown down on the issues was like "trying to nail a raw egg to the wall."

But in the strange campaign occurring in California, there has been little effort to pin Brown down on anything. In fact, Younger presents the unusual spectacle of a candidate trailing in the polls who is trying to duck a televised debate with the front-running incumbent.

Negotiations for a debate have been stalled for weeks, chiefly because of Younger's refusal to appear with Brown before a panel of reporters as has been customary in California campaigns. Younger insists he will go on only head-to-head with Brown without reporters participating.

Some see this as an unwillingness of the 60-year-old Attorney General to be visually matched up with the 40-year-old Brown.

Whatever the reason, Brown was busy this week once more in trying to make a virtue out of a supposed political liability. In a new television commercial Brown's voice is heard saying, "People ask me, why do I change my mind?"

The answer, also by Brown: "Well, when I find out something doesn't work, then I try something else."