It's now neck and neck for Hart and Mondale in California

Northern Californians may be outnumbered, but their sensibilities have been known to influence the state beyond the strength of their numbers. So, their apparent preference for Gary Hart may play heavily in Tuesday's primary.

In the 1972 primary, for example, a solid bloc of support for George McGovern in northern California was enough to hand him the state despite a wide margin of support for Hubert H. Humphrey in Los Angeles-Orange County population center, pollster Mervin Field recalls.

And in an important political contest in 1982 - the proposed Peripheral Canal project that would have diverted more water from the north for development in the south - the pattern was similar. Northern California opposition prevailed, despite the electoral balance that tips in favor of the sunnier south.

The northern California vote can be powerful when the electorate is divided elsewhere in the state. But the north's propensity for voting as a liberal bloc is not easy to justify because of the diversity of the electorate here, says Hubert Marshall, a political science professor at Stanford University.

''The differences between northern California and southern California are real, not imaginary,'' Dr. Marshall says. But they appear only to be political differences because each region has a proportionate share of wealth, minorities, and business diversity, he says.

Between the rich furrows of the Central Valley and the wooded green that stretches north to Oregon, there is an electoral diversity that includes Marin County ''limousine liberals,'' Silicon Valley technocrats, Mendocino loggers, conservative central valley farmers, and a Bay-area ethnic jumble of blacks, Hispanics, and Asians.

By all counts northern California should be ''prime Hart territory,'' says Richard Brody, a Stanford political scientist. Indeed, Mr. Field's latest California Poll shows Senator Hart leading Walter F. Mondale by 7 percentage points in northern California, while support is evenly divided statewide between them.

Northern Californians are sensitive to environmental and high-technology issues. Indeed, the state's north-south split is usually characterized by the image of the north thumbing its nose at the social and economic excesses of its southern neighbor. Wilderness and coastal areas in the north are untouched, and many people choose to live in this environment rather than in the more congested south. To these residents, Hart's ties to the environmental movement, which has a large base of operation here, are appealing.

While Mr. Mondale has party support in San Francisco, his union ties won't sit well among the young professionals of Silicon Valley, who are more likely to lean toward Hart's free-enterprise and New Age rhetoric.

The Rev. Jesse Jackson is expected to do well in the Oakland and Berkeley minority communities - largest outside Los Angeles in this state.