Ronald Reagan continues to hold a decisive lead over President Carter in California, but his own popularity has declined and the lead now appears to be based on dislike for the president and the continued drawing power of independent John B. Anderson.

"What this means is that the race is almost by definition a negative one," said one well-place Democrat. "Reagan is a controversial figue and Carter has never been popular in California. The two campaigns have to proceed from this premise."

The last California pool, conducted by Mervin Field, showed Reagan with a 10 percentage point lead over Carter, but surveys of two California congressional districts, made available to The Washington Post, show both men with low favorability ratings.

A Republican survey in one district gave Reagan as many unfavorable as favorable responses. Carter fared even worse, with a 5-to-3 unfavorable rating. A Democratic survey in another district, using a slightly different measurement of favorability, found Reagan with a 6-to-5 favorable edge and Carter rated unfavorable by a similar ratio.

The figures in the lates Field poll, taken in early September, were Reagan 39, Carter 29, Anderson 18, undecided 10 and other candidates, 4. This was down from a 19-point lead which Reagan held over Carter in a July poll but it was about the same lead Reagan held in earlier Field surveys in April and May.

Field found in his latest poll that the Anderson voters would go to Carter by a 9-to-5 ratio in a two-way race and that Reagan's lead would be six percentage points if this happened. Carter lost California by only two percentage points to Gerald Ford in 1976, but the president's favorability ratings generally have been lower here than in the rest of the nation.

The Carter campaign in California has operated with two objectives. The first, which has always seemed improbable, is defeating Reagan on his home turf and winning this most populous state's block of 45 electrocal votes.

The second objective is what some Democrats call "a strategic diversion" that will force Reagan to devote additional time to California and reduce his campaigning in closely contested Northeast and Midwest industrial states where the election is likely to be decided.

"If we can bring him back to California for one extra day, it could make the difference in Ohio or Michigan," said one Democrat who privately acknowledges that Carter is unlikely to win California.

Jack Courtemanche, Reagan's California campaign manager, has labeled this tactic a "smokescreen" and predicted that it will not succeed.

Reagan will campaign in the Los Angeles area Friday and remain the weekend at his Pacific Palisades home before resuming campaigning in the West.

A reagan campaign source said that the former California governor will make one more trip to the state before Nov. 4 election. But the Reagan campaign schedule for the final two weeks of October remains highly tentative so that the GOP nominee can campaign in close key states.

The Carter campaign has been concentrating its September efforts on registration, emphasizing efforts among minority voters. One Democratic campaign official said that the results of the registration drive in the black community have been "disappointing."

But the Carter effort is considered much better organized here than in 1976. Unlike four years ago, the campaign is relying on the advice of experienced, in-state political professionals such as state Treasurer Jesse Unduh.

The Carter campaign is being managed by Mickey Kantor, a knowledgeable Los Angeles lawyer who has in the past managed the campaigns of Sen. Alan Cranston and Gov. Edmund G. (Jerry) Brown Jr.

When Carter appears at a Los Angeles fund-raiser later this month, the proceeds will be earmarked for the state party, as California Democrats long have desired. This will mean additional candidates and Cranston, which the Democratic strategists hope will produce some "reverse coattails" for Carter, who lagged behind most other Democrats in 1976.

Republicans will concentrate on voter turnout efforts rather than registration. Depsite Reagan's relatively low favorability, he has a solid base of support among Republicans and independents who approve of his record as governor.

"The real edge that Reagan has here is that his favorable voters are highly favorable -- they have been voting for him for 15 years and are proud he's running for president," says a Democratic campaign worker. "Our people don't like Reagan, but it's going to be an effort to get them to the polls."