With just over a week to go, Brown-Wilson race is a tossup

If you recently heard a grinding sound somewhere to the West, it may well have been California Gov. Jerry Brown's political bandwagon getting back into gear.

Or it could have been San Diego Mayor Pete Wilson gnashing his teeth.

Only the most partisan backers of Democrat Brown are flatly predicting he will defeat Republican Wilson in the US Senate contest Nov. 2. But most polls indicate the Brown-Wilson race is a dead heat just over a week before election day.

That's a far cry from the 20-point lead Wilson was said to have just after he and Brown were nominated by their parties in the June 8 primary. The margin has fallen steadily: It was 11 percent by Sept. 1, about 4 points by mid-September, and a tossup in the first week of October.

An early October voter sampling by Mervin D. Field's authoritative California Poll showed Brown with 46 percent and Wilson 45. Most of the remaining 9 percent were ''undecided,'' with a smattering of votes going to minor candidates. (The opinion sampling's error margin was 3.7 percent.)

A Los Angeles Times poll later in the month showed Wilson with a slight lead - 46 percent to Brown's 41 percent.

Whether or not Brown wins this election, he has come a long way out of the political shadows that hung over him in late 1981. Even if California Republicans succeed in ''retiring Jerry Brown,'' as they vowed to do this year, it's not likely to be a permanent retirement.

Many Democratic voters who spurned Brown in the June primary are behind him in late October. Major Democratic donors and groups such as labor unions and the California Teachers Association never abandoned him. This was evident from the ease with which he built a campaign treasury that has enabled him to outspend Wilson.

But not by much. Between them the two major California candidates for the Senate will expend more than $15 million - mostly on television commercials.

Republican Wilson's fund-raising program was shaky in the primary season, but once he won the party nomination, both state and national sources came through handsomely. A late August appearance by President Reagan at a Los Angeles fund-raiser reaped $1 million for the Wilson coffers. But an expected second appearance by the President on behalf of Wilson apparently will not take place. Wilson's explanation is that Reagan has ''concluded he can do more good other places, and that we can win without him.'' Some political observers speculate the reason may be concern that a California appearance by the President at this time would not help Wilson and could even cost him some votes.

Although other US Senate races are of more immediate import in Washington, a great deal of national attention is drawn to the California contest, and with good reason. Election of Mayor Wilson as the replacement for retiring Republican US Sen. S. I. Hayakawa would be seen as a moral victory, at least, for President Reagan. It could also be called a morale booster for the President, who has told Californians, in so many words: ''Anybody but Jerry Brown.''

And, of course, it would help the GOP to keep its slim Senate majority without having to pick up a new seat in another state.

Conversely, a Brown victory could be part of Democratic return to control of the Senate - or at least a weakening of the Republican leadership position.

Of greater long-term significance, election of Brown to the Senate would place an impressive brace of California Democrats in that exclusive, 100-member club, which is a spawning ground for presidential candidacies. Sen. Alan Cranston would be the senior Democratic partner, and at this point it is he who seems almost certain to seek the party's presidential nomination in 1984.

Governor Brown has eschewed presidential ambitions in the near term and endorsed fellow Californian Cranston's White House aspiration. But few doubt that the 44-year-old Brown harbors long-term designs on the presidency. A six-year term in the Senate would provide a politically maturing Brown a much more credible medium for a White House bid - say, in 1986 - than his improvised attempt in 1976 and the damaging one in 1980.

In a recent Los Angeles appearance, Brown admitted: ''I used to sprint for a few higher offices that I found were not ready for me, and that's when I became a long-distance runner. . . . But I'll get to the goal eventually.''

Should Mayor Wilson win, he would be a Reagan team-player in the Senate - basically. But he would not hesitate to speak - and vote - his own convictions, as evidenced by his departure from the White House line on some issues during the campaign. Most prominent, and perhaps most costly in terms of a certain coolness at 1600 Pennsylvania Avenue, was his refusal to support the President on recent tax increases.

How the 9 or 10 percent of ''undecided'' voters feel about some half-dozen major issues is the likely key to victory on Nov. 2. The California Poll found that inflation, crime, unemployment, taxes and government spending, energy supply, and social security were the most important, in that order, to Californians it questioned. Other poll results indicate that voters concerned about four of those issues are more likely to vote for Brown.