In a key California county, the race looks lopsidedly close. GOP will win Orange County, but the margin is key to Dukakis

Orange County, one of the country's purest founts of right-wing Republicanism, may hold the key to who wins California this fall - and the White House. No, Michael Dukakis, the Democratic standard-bearer, is not going to carry the county that served as the wellspring of the Reagan Revolution and where Lt. Col. Oliver North is still as popular as tanning oil.

The question is how much - or little - he will loose by in his contest with George Bush.

Harboring the third largest number of registered voters in California, Orange County is always critical in determining who will carry the nation's most populous state.

Republicans depend on it to offset the vote in more Democratic northern California, particularly San Francisco. The rule of thumb is that if the GOP carries Orange County by a wide enough margin - 150,000 to 200,000 votes - it usually wins the state.

The Republican faithful here think they are poised to do just that. The Democrats see a few openings for themselves.

Thus, a spirited battle is shaping up in 1988 in this most Republican of strongholds, certainly more of one than has occurred in the recent past, when Ronald Reagan, the favorite son still affectionately referred to here as ``the governor,'' topped the GOP ticket.

``I think it is the real key to the state,'' says Mark Baldassare, a sociologist and pollster at the University of California, Irvine.

Several trends are at work that could affect the outcome. For the Republicans, one of these trends - voter registration - is heartening. But another - voter turnout - the GOP finds more harrowing.

Although this prosperous county south of Los Angeles has grown dramatically - doubling in population in 15 years - its Republican hegemony shows no sign of waning.

Some argue that the county isn't as reflexively conservative as it once was, but it is still solidly GOP: Republicans maintain more than a 200,000 edge in voter registrations (584,000 to 382,000).

The county's congressional and state legislative delegations are all Republican, and one of the area's most visible mayors, Dan Young of Santa Ana, recently switched from Democratic to GOP ranks.

``We feel we are in the best shape we have ever been in,'' says Tom Fuentes, Orange County Republican chairman.

Yet the GOP belt isn't cinched completely tight. Stephen Teichner, an Orange County-based pollster, says the conservative wing of the party hasn't turned out in large numbers the past two years. Coupled with some of their members' past distrust of Mr. Bush - as well as the lack of any statewide ballot initiative likely to bring them out - this could make conservative turnout a challenge again.

``What we know is that it is not going to be the easy cakewalk for the Republicans that the registration numbers would suggest,'' Mr. Teichner says.

The Democrats, who were thumped by more than 400,000 votes in 1984, are not ceding the county. This was evident last week when Governor Dukakis, in standard button-down attire, shook hands with a surfer on the sand at Laguna Beach.

True, salt-scented Laguna is one of the few Democratic strongholds in the county - or, as GOP officials derisively refer to it, the home of ``limousine liberals.'' But it was Mr. Dukakis's third visit to the county, more than any other Democratic nominee in modern times.

The party will be opening a number of headquarters around the county for the first time since 1972. Democratic officials say that Dukakis's image as an efficient manager and his environmental stands will play well with registered Democrats, of which there are more here than in San Francisco, and some moderate Republicans.

``We are going to fight for every vote,'' says John Emerson, a senior adviser to the Dukakis campaign in California.

The Republicans are not taking their base for granted. Bob Dornan, a conservative congressman from Orange County who has been a chief surrogate for Bush, will give one of the nominating speeches at the GOP convention next week. The vice-president spent the night of the California primary in June here, and he and his family are expected to visit numerous times before November. The local party's chief icon, Ronald Reagan, will be stumping in the area.

Unknown is how much the county's changing nature will affect the voting. Although still sleepy and suburban in comparison to garish Los Angeles, Orange County long ago outgrew its image as the home of Disneyland and palmy planned communities. It has developed into a vibrant economic entity, attracting new industry and wealth (median income: $44,000).

A slow-growth movement has taken hold, but analysts doubt it will influence the election.

The Republicans think they can combine enough factors to deliver a 200,000-vote margin in the fall - and thus the state. The Democrats think they can hold the gap to 150,000 votes.

In other words, in its own lopsided way, it looks like a tight race.