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

A political state unto itself, the nearly endless urban expanse of Los Angeles is the knot in a state that is otherwise mostly Hart country. But with more than 7.5 million people, Los Angeles County is enough of a knot to throw the whole state up for grabs in next Tuesday's primary.

The southern California political patchwork is so vast and complicated, so little controlled by political parties or labor unions, that politicking has long since been a matter of putting images across on television and radio.

On the airwaves, the campaign is a much simpler affair. The Los Angeles media market reaches 40 percent of the state's audience, and 30 seconds of prime-time, major-station TV costs $15,000.

Between Gary Hart and Walter Mondale, Los Angeles is nearly the mirror image of the rest of the state. A Los Angeles Times poll taken last week showed Mr. Mondale with twice the support of Senator Hart in this county, while Hart was 50 percent ahead of Mondale elsewhere in California.

''The county,'' says Larry Berg, director of the University of Southern California's Institute for Politics and Government, ''is probably a little less 'yumpie' (young upwardly mobile professional) than people think it is.''

A recurring motif in the Los Angeles sprawl is the 1950s-vintage neighborhood of ranch-style houses, often with campers or vans in the driveway for weekends in the desert. Many of these are blue-collar, union households.

Although unions don't have the strength here that they have in the East, Los Angeles has a fairly traditional Democratic base in blue-collar workers and ethnic minorities.

On the west side, entrepreneurial values rank high, observes political consultant Peter Coye. And by this measure, Hart's image is that of the political entrepreneur in contrast to Mondale's political bureaucrat.

Hollywood has been Hart's from the start. Young (or young-at-heart) celebrities gave him financial backing when few voters had heard of him.

The east side is Mondale country. Here almost half of California's Latinos are concentrated. Mondale has a lock on the key Latino endorsements (except that of Cesar Chavez, who is for Jesse Jackson). Most expect him to carry it.

Latinos could be more important for Mondale than their numbers imply. Hispanic districts have been loyally Democratic in their voting patterns, so they have been allotted a correspondingly high number of delegates.

South-central Los Angeles is black, from poor Watts to bourgeois Baldwin Hills. Here Jackson and Mondale are locked in battle. Three districts are expected to go to Jackson, but Mondale retains his hold on the black establishment.

Jewish Los Angeles is concentrated in the Fairfax district. From here stretching west is a community of elderly Jews considered to be the kind of constituency that put Mondale on top in New York.