The California legislature and Gov. Edmund G. (Jerry) Brown Jr. are learning the hard way that it is easier to vote against the New Deal than to repeal it.

While Brown and legislators of both parties have responded vigorously to the tax-cutting mandate of Proposition 13, they are finding that such social services as welfare, medical care and enviromental protection are deeply entrenched in state and federal law.In contrast, such popular services as police and fire protection are not mandated and are easily slashed.

An in-depth survey taken by Merrin Field, whose polls accurately forecast every contest in the June primary, found that Californians had an overwhelming proference for reducing public assistance programs if cutbacks are needed because of Proposition 13.

Sixty-two percent of California voters favored cutting back on welfare programs. Other prime candidates for cuts were government-backed public housing projects; (41 percent) and enviromental protection regulations (24 percent).

At the other end of the preference spectrum, only 6 percent of voters wanted to cut fire departments and only 8 percent wanted to reduce police services.

Such mandates are not easy to ignore, no matter what the law. Legislators have responded to the voter mood by trying to freeze cost-of-living increases that welfare recipients would have received this year. But evenr this relatively modest action will require repeal of a separate state law that mandates the increases.

Even if the freeze sticks, the state will simply be holding the line on welfare programs while some police departments are cutting back by 10 percent.

A large part of the difficulty, for those who want to cut welfare, is the complex interrelated set of laws and regulations that has grown up in the nation's industrial states ever since the beginning of the New Deal in 1933.

Eligibility standards for the most expensive public assistance programs - aid to dependent children and medical care, plus aid to the aged, blind and disable - have been largely determined in Washington and imposed on the states. In California, these federal regulations have in turn been imposed on the counties, which could lose federal and state funds if they decline to carry out the mandates from above.

No California county, especially after Proposition 13, could survive such a loss. Of the $5 billion spent on public assistance in California, half comes from federal funds, with the remaining costs divided about equally between counties and the state.

Recognizing that the drain on county property tax revenues had made welfare recipients the chief target of middle-class voters, the legislature decided yesterday to have the state pick up the county costs as part of the package to provide local governments with assistance in the wake of Proposition 13.

The web of welfare regulations is not the only problem. Public pension plans, many of them approved by the boters, have grown with such leaps and bounds that thye have eaten up a gigantic share of local government revennes.

In Los Angeles, pensions for retired police and firefighters accounted for nearly one-third of all money collected from property taxes in 1977.

Pensions also are the largest item in San Francisco's budget. A San Francisco police officer or firefighter making $19,000 who retired this year at 50 after 20 years of service would receive $14,300 a year plus cost-of-living increases for the rest of his life.

Courts repeatedly have held that such pensions are binding contracts that cannot be reduced. The alternative to cutting pensions is cutting police officers, which many California cities are now doing by attrition.

Even enviromental protection regulations, a comparatively new thread in the social service fabric, won't be easy to eliminate. In fast-growing Orange County, for instance, developers already are complaining about proposals to eliminate planners needed to process federal and state environmental impact reports.

Instead of eliminating them, Orange County is moving to charge developers for their services, a recognition that both the planners and the environmental regulations are here to stay.

Here to stay, too, are many of the programs about which voters appeared to be complaining when they approved Proposition 13. Public officials at all elevels in California are learning that a complicated edifice of social services that has taken nearly 50 years to build does not come toppling down in a day.