"I feel very good," said Gov. Edmund G. (Jerry) Brown Jr., breakfasting with reporters recently. I feel unaccountably good."

Brown unaccountably has been almost euphoric in the closing week of the California Senate campaign despite the latest California Poll, which shows him trailing his Republican opponent, San Diego Mayor Pete Wilson, by 6 percentage points. Wilson has 47 percent, Brown 41 and 12 percent are undecided or supporting minor party candidates, according to the poll.

Last June, Wilson enjoyed a comfortable 22-point margin, but the previous California Poll in early October showed him trailing Brown by 1 point.

The race is one of the most expensive in history. As of Oct. 12, Wilson had reported spending about $5.1 million and Brown more than $3.9 million.

Wilson, meeting with the same breakfast group one day later, displayed a low-key demeanor markedly contrasting with Brown's high spirits.

He said that his campaign's nightly tracking polls show him with a lead "consistent with the public polls" of between 6 and 11 points and that they show little or no movement toward or away from either candidate. He would say little else, however.

Wilson, who describes himself as "cautiously optimistic that I am going to win," is sitting on his lead.

He and his managers needed no reminder that it was in talking with the same group about voter ratification of California Supreme Court justices that Wilson allowed himself to be backed into a verbal corner in which he approved election of federal Supreme Court judges. The Wilson forces were determined not to give Brown any last-minute openings.

Asked for a response to Soviet President Leonid I. Brezhnev's speech last Wednesday calling for a strengthening of Soviet military forces, Wilson declined to comment on grounds that he was unaware of Brezhnev's statement, which at the time had been on the front page of every major California newspaper for two days.

Wilson attributed his lack of knowledge to the fact that "I have been on a merry round, entertaining people all over the state. Campaigning is not a normal activity."

Brown is pinning his hopes on what he believes is a large and volatile undecided vote in an election that will be very close.

"I have some baggage they the undecided voters are not quite ready to accept," he concedes.

Brown has tried to make the national economy the main issue of the campaign.

The Senate election is a chance for voters to "send Mr. Reagan a clear message, so that he will take a second look . . . . I'm asking the voters to vote their pocketbooks and not stay the course but change the course," he said.

The Brown-Wilson race has been described as a "tennis match in a gutter."

It left the high road with a Brown commercial accusing Wilson of not supporting the nuclear weapons freeze proposition on the ballot. The advertisment featured a little boy saying he wanted to "go on living," a nuclear mushroom cloud and the admonition, "Vote for your life. Elect Jerry Brown to the U.S. Senate."

Wilson in turn has a commercial that plays up the relationship between Brown and Tom Hayden, the former student activist running as a Democratic Assembly candidate, and Hayden's wife, Jane Fonda.

"Hayden and Brown," an announcer intones, "Brown and Hayden. It wasn't good for California, it wouldn't be good for this country."

A Brown ad shows fanciful blowups of checks made out in the amount of "None and 00/100s." Would-be recipients of the checks are Wilson's landlord and the Internal Revenue Service.

One campaign issue has been that Wilson, since his divorce, has lived rent-free in apartments owned by friends who Brown claims do business with the city of San Diego. Another is that Wilson paid no federal income tax in 1980 because of a tax shelter Brown characterizes as a "scam."

The tag line on one commercial: "Jerry Brown isn't perfect, but he pays his own way."

Brown recently opened his modest $400-a-month Sacramento apartment, which he pays for himself, to reporters and photographers for the first time. Was it a campaign stunt to keep attention on Wilson's rent-free living arrangements? "Yes," Brown replied cheerfully.

But the major factor may be what pollster Mervyn Field calls "just too heavy of a negative" in Brown's image.

"I don't believe there are any individual issues," Brown says of his negative ratings. "I believe there is a negative residue of the controversies, the problems I had running for president last time -- the skepticism, the derision, the controversy that came out of that -- the fighting with the legislature a couple of years ago, the Medfly. These are not so much substantive issues but just a negative impact that has been left."

Does he regret his runs for the presidency? "I don't regret 'em now. Ask me on Nov. 2," he said.