Close race in California 'Bible belt.'

At some point in driving east out of the Los Angeles basin, a top-40 radio station sputters and cuts out, giving way to country-and-western fare. Lionel Ritchie segues to Tammy Wynette.

The politics shifts along much the same lines.

Here in the so-called Inland Empire around the desert cities of San Bernardino and Riverside, the 36th Congressional District is a sort of southern California Bible belt, mostly Democratic, but Southern-style Democratic with roots in conservative Protestant churches.

This is one of only two races for Congress in California, and one of few in the entire West, that is considered a close contest. As a result, Rep. George Brown Jr. and his GOP challenger, John Paul Stark, are both attracting funds from politicians and interest groups nationwide.

Congressman Brown - a 20-year veteran in the House - is a liberal's liberal, a self-described peacenik with strong outside support from environmentalists. His opponent, Mr. Stark, is a conservative young Christian fundamentalist, an insurance agent and former manager with Campus Crusade for Christ International. He counts the US Chamber of Commerce among his contributors.

This race matches up two of the major traditions of religion in American politics.

Stark has a natural base of support in the conservative churches. Many of the voters here are emigres from the South, and belong to fundamentalist congregations. Others belong to long-established Mormon and Seventh-day Adventist communities here. Generally, they are culturally and politically conservative.

Brown, on the other hand, has been preaching the social gospel of liberal activism more familiar in the mainline Protestant denominations of the Northeast. A Methodist with Quaker upbringing, he recently told a Methodist congregation that welfare and opposition to nuclear weapons are as ''pro life'' as opposing abortion.

This is the third time Brown and Stark have faced off. By the usual measure, this should not be a winnable district for a Republican. Democrats outnumber Republicans nearly 2 to 1. But Ronald Reagan beat Jimmy Carter here in 1980 by a landslide, and Brown beat his then-unknown opponent by only 53 percent to 43 percent.

In their second match-up in 1982, Brown outspent Stark by double, and was boosted by a redrawn district that added Democrats to the mix. Yet the challenger managed to close the gap by a couple of points.

This year Stark, who has grown steadily better-known in each election, has the advantage of running on a ticket headed by a popular President. And for the first time, Stark and Brown have comparable campaign budgets, a little more than

Stark now considers himself ''within striking distance.''

This is a blue-collar district. Many people here have been squeezed from the Los Angeles basin in search of housing they can afford. In fact, the region has become one of the fastest-growing areas of the state, steadily becoming a part of the Los Angeles megalopolis.

It is also where the sea breeze blows most of the Los Angeles smog, trapping it against the mountains. Stringfellow Acid Pits, a badly leaking former dump for toxic wastes, is also in the district. So local environmentalism is not of the philosophical sort, but reflects immediate grievances.

National security is also a tangible subject here. With two Air Force bases and a good share of aerospace and defense-related industries, many paychecks are tied to defense spending.

On defense issues, Stark calls Brown ''left wing'' to the point of irresponsibility. ''He seems to have gone off the deep end in certain areas, saying some things a congressman shouldn't say.'' Brown votes against many defense projects, yet he says he works hard ''to keep our military bases healthy.''

''I even went so far as to change my vote on the B-1 bomber,'' a project he says is very hard to justify militarily, ''because it would have a major economic impact on my district. I'm a pragmatist.''

Brown has a base of support in the nearly a quarter of his district that is Hispanic, and votes heavily Democratic. About 7 percent of the voters here are black and, with the exception of some of the conservative church congregations, are solidly in the Democratic fold.

But, he says, ''We're also fighting for the same votes Stark thinks he's got a lock on, very conservative Moral Majority people and millenarians.'' (The latter group believes that the world will soon end.)

But Stark - who supports constitutional amendments for a balanced budget and school prayer, opposes gun control, and notes that, unlike Brown, he gets no money from labor, gay-rights, or peace groups - may be preaching a political sermon more to suited to their taste.