Baltimore County, a massive horseshoe of land surrounding the city of Baltimore, is a mixture of steel plants and vast country mansions. Its 655,000 residents include silk-stocking Republican bank presidents, blue-collar Democratic immigrants, typical suburbanites, middle-class Jews and a small number of blacks. Its electorate is huge, varied and, most important, unpredictable, making Baltimore County a nerve-racking wild card in Maryland elections.

Stretching over 610 square miles from Pennsylvania on the north to the port of Baltimore on the south, Baltimore County might best be described as Prince George's and Montgomery counties merged into one. Like Montgomery, it has pockets of conservative rural wealth, the sort that dominated the county before the urban center began to explode outwards 20 years ago. Like Prince George's, it has an industrial core that is heavily blue-collar, heavily Democratic, and socially conservative. Like both, its biggest employer is the federal government, which houses the Social Security Administration's headquarters in Woodlawn.

But Baltimore County is unique in Maryland, and it holds a special place in the political process. Its legacy goes deeper than its tainted years of corruption (scandals ended the careers of many leading county politicians, including former county executive, governor and vice president Spiro Agnew). The county has a reputation for providing the biggest, and most erratic, turn-out of voters in the state. While statewide candidates look to the Establishment in Baltimore for money, they look to Baltimore County for votes.

Four years ago, for example, the county gave winning gubernatorial candidate Harry Hughes 20 percent of his statewide total. As the Nov. 2 general election approaches, the four major state-wide candidates -- Democratic incumbents Hughes and Sen. Paul S. Sarbanes and their GOP challengers, Robert A. Pascal and Lawrence J. Hogan -- eye it warily, with a combination of fear and high hopes.

Although polls released today by two Baltimore newspapers show the county solidly in the Democratic camp, one of them shows Pascal gaining dramatically on Hughes. And Hogan, although behind, is encouraged by signs that show him appealing to some segments of the large blue-collar vote. In 1976, Sarbanes carried the county by only about 20,000 votes.

"The county has a tendency toward fickleness that is frightening," said former county executive Theodore Venetoulis, who ran for governor unsuccessfully in 1978. "It tends to be very moody, very faddish, very fickle. You never know what is going to turn the voters."

Although Baltimore County, like the city, is overwhelmingly Democratic, it is more conservative in voting habits. Its voters, by some estimates, are parochial in local elections: They vote for the names they know and the people they like, putting party affiliation and ideology aside. But in national races they focus on issues and are decidedly more conservative on economic and social matters. Only twice in the last century have voters chosen Democratic candidates for president, despite giving overwhelming margins to the party in elections for the county government and the General Assembly.

In wooing voters from the sprawling county, unlike the city, candidates for statewide offices can not count on a cohesive set of institutions that guarantees votes, money and political muscle. The county has a political base on the east side but, whereas the city has the Establishment, a powerful community of bankers and lawyers (many of whom live in the county) who provide the financial clout for campaigns, the county has virtually none. And it is influenced much less than the city by the personage of Baltimore's mayor, William Donald Schaefer, or by the editorial pages of the Baltimore Sun.

There is another missing ingredient in Baltimore County -- the powerful black vote -- and its absence is no accident. While the population of the county exploded in the 1960's and 1970's, bringing more immigrant families from East Baltimore to Dundalk and Essex, more Jewish families from northwest Baltimore to the Pikesville, and more wealthy city folk north to "the Valley," the black population increased only slightly, from about five percent to eight percent overall since 1970. Most of the black population is concentrated in the Liberty Heights area that extends from the western edge of the city. Many of the black families that moved were middle class.

Baltimore County historically has been less than hospitable to blacks. In 1970 the U.S. Commission on Civil Rights selected the county as a prototype of a white Eastern suburb ringing an increasingly black city. In the 60's and '70s, county officials were accused of planning a "racial wall" around the city. The county, until a few years ago, rejected federal aid for urban renewal and low-income housing, which some observers said was a sign of its stubborn resistance to an outmigration of blacks from the inner city. In the mid 1970's, the county council, in a rare move, rejected the county executive's choice for administrative assistant, a man whose wife was black. The county accepted only two of three proposed mass transit lines that were to extend from city -- the one rejected would have been a light rail system running out from the predominantly black east side of Baltimore.

"We were very careful with the race issue, because it can destroy an administration," said one former high-ranking county politician who believes that racism still prevails in many areas of the county and explains the county's tendency toward conservatism. "We never fought for urban renewal. We just wouldn't do it. We did a lot of symbolic things, like increasing the number of blacks in the police and fire departments. We did things that didn't require battle."

For the gubernatorial or U.S. Senate candidate trying to court voters in Baltimore County, the first keys to success are the densely populated east and west sides and the Democratic organization of gregarious Donald Hutchinson, whose position as county executive gives him automatic status as one of the state's most important politicians.

In the blue-collar, ethnic strongholds of Dundalk and Essex on the east, where unemployment now stands at nearly 18 percent, and where shipyard cranes and the massive Bethelehem Steel plant loom over the waterfront, the county's first political machine was born. Immigrant families, many of them German and Irish and Polish and Lithuanian, settled into scores of closely knit communities, marrying and intermarrying, creating the ideal base for political organization. For the most part they are devoutly Catholic, and devoutly Democratic.

"If you asked my constituents what nationality they are, they would probably reply, 'Democratic.' " said state Sen. Dennis Rasmussen, from Essex, whose family name, like Hutchinson's, is synonymous with east side politics. "People aren't concerned with liberal or conservative labels. They want their elected officials to be visible. Our office is open five days a week. We get involved in all aspects of daily living."

Over the years, the legendary machines of former executive Michael Birmingham and former state Sen. James A. Pine were born and prospered on the east side until they were toppled by scandals and a zealous "reform" movement launched in 1974 by former county executive Venetoulis, who was backed by younger members of the old east side families, including Hutchinson and Rasmussen.

Now the system has changed, with volunteers replacing machine politics. But the patchwork neighborhoods, the interlocking families, the old political names -- Hutchinson, Rasmussen, Connelly, to name a few -- are still the bases of power. And the local pols' ability to mobilize votes for other Democratic candidates is what gives the east side its political clout today.

"I have several hundred cousins, my mother has 13 brothers and sisters, and my father has seven brothers and sisters," Rasmussen said, explaining how family connections play a role in east side politics. "After two or three generations that is going to be a lot of intermarrying. I know a vast majority of the people in my district, or they know me."

Hogan, the Irish-Catholic Republican county executive from Democratic Prince George's, thinks he can make inroads in southeast and southwest Baltimore County, in Dundalk, Essex, Catonsville and Arbutus, despite the 7 to 1 voter registration favoring Democrats. So does Pascal, whose campaign stresses his Italian background. Hogan is banking on the blue-collar worker's conservative bent on issues such as abortion, prayer in school and U.S. military strength to outweigh the area's traditional Democratic thinking on the economy. In his visits to beer halls and American Legion functions, Hogan sells himself as "strong on defense" and "tough on crime."

But the local politicians are not impressed, and continue to view the east and west sides as a core of support for Sarbanes, despite a right-wing media campaign that has portrayed him as "too liberal," and Hughes.

"Hogan is blowing in the wind," said Hutchinson who, with one of the best organized political operations in the state, is leading the Sarbanes and Hughes campaigns in the county. "He can go down there and demagogue and play the abortion issue all he wants. But people are worried about jobs."

Venetoulis, whose own political fortunes sank in the county in part because he was perceived as an outsider (from Baltimore city), said of Hogan's effort: "A candidate from Prince George's has to understand the difference here in Baltimore County. Give me an industry in Prince George's that compares with Bethlehem Steel. It's a different kind of blue-collar sensitivity. They Baltimore Countians don't know the Washington suburbs from nothing. They own their own homes, they're Orioles fans, and they've lived there forever."

If there is a check on the powerful east and west side political organization, and an area of growing interest to state-wide candidates, it is Pikesville. This middle and upper middle class, predominantly Jewish enclave extends from the northwest side of the city and provides the heaviest voter turn-out in Baltimore County.

The area is also parochial: Its voters are progressive and Democratic on local and state issues, and more conservative on the economy and on foreign affiars, where the security of Israel is a key concern. In last month's primary election, Pikesville gave Sarbanes' his biggest margin in the county.

"Hogan will need binoculars to see how far back he comes in this district," said Pikesville's Demcoratic state Sen. Melvin A. Steinberg, who is hoping to ascend to the senate presidency if reelected in November. "People in my area are hesitant to register Republican because they remember the Depression days. It is impractical for them not to go Democratic." The Republican Party

To the north and northwest of the city, beyond Towson and the Baltimore beltway, near Timonium, Cockeysville and Monkton, is the county's only real pocket of Republicanism, "The Valley." It is an area of mansions and country estates, many set on pictorial landscapes. There are exclusive country clubs and annual hunting events that draw the financial, political and social elite out from the city.

Valley residents want lower property taxes and curbs on development (the most recent controversy was whether an outdoor pavilion should be built for the Baltimore Symphony in a scenic park called Oregon Ridge). They vote Republican consistently in local and national elections.

But the Republican Party has been unable to capitalize on the area's wealth, or to use it to expand the party's base elsewhere in the county.

"For the last 20 years there has been a real lack of cohesiveness," says a Republican active in county politics. "The problem is that each Democratic faction has greater numbers than the whole Republican party."

County politicians say the GOP squandered a chance to make inroads in the mid-1970's, when reform Democrats, not Republicans, capitalized on the corruption issue and ousted the Democratic Pine machine.

Now the county's most successful Republican politicians have either withdrawn from politics, switched parties, or departed for other jurisdictions. Hogan and GOP gubernatorial nominee Robert A. Pascal, who come to the county to campaign, have found a vacuum in their party.