Here in Calvert County, a long thin finger of land in Southern Maryland, it is the best of times and the worst of times.

Ever since the big new nuclear power plant at Calvert Cliffs quadrupled tax revenues overnight in the 1970s, from $7 million to $28 million, the county government has been booming. Just the other week, several county offices moved into remodeled quarters that once housed Goldstein's department store across from the old Courthouse. From Solomons in the south to Dunkirk in the north, there are new schools, parks and libraries to serve the people.

There is also, in this still largely rural region, 18.1 percent unemployment, according to the latest available figures--the fourth highest in the state and the worst within an hour's drive of Washington. The number of unemployment checks mailed the other week was up 60 percent over the same time a year ago.

"We live in a county," said Thomas Axley, an attorney active in the Board of Trade, "that's government-rich and people-poor." A single vacancy for a county telephone dispatcher recently drew 54 applicants. After the power plant, the largest single private employer here is McDonald's: More than 200 applied for its 75 jobs when the fast-food restaurant opened last fall.

Even in winter, when construction layoffs are common and when the deep freeze keeps watermen from working, the 18.1 percent unemployment statistic is, as a local newspaper noted last week, "unparalleled around the region."

With spring in the air and tobacco seeds freshly planted in the ground, normally come a drop in unemployment and a rise in spirits and hope for better times. This year, however, the hope is tempered by the times, and the seasonal cycle that grips the countryside seems harder to break.

The dozen or so empty storefronts that dot the county's few shopping centers contribute to the malaise many say they feel. No matter that one shopping center owner promises new tenants soon will fill the vacancies, the failed businesses seem to speak louder in a county of only 34,000 citizens.

When a real restaurant closes its doors, as did Rusty's in the Calvert Village Shopping Center a few weeks ago, it is disturbing news. And when the county loses its only shoe store and the carwash around the corner goes bankrupt, concern can turn to despair.

Dorsey Gray Jr., whose father started selling Fords here in 1919, is hanging on, barely. Without the bank's beneficence, he concedes, he could be bankrupt, as was his father during the Depression. "Damn worst I've seen since I've been in business," he said. "I'm next to bald-headed now."

Bill Gray, an out-of-work construction worker and no relation to Dorsey, passes part of his idle days at the car dealer's. "I'm so poor, they won't even give me food stamps," he said. It was meant to be a joke. Gray, 41, worked last in December and draws unemployment, as does his wife, who was laid off Jan. 16 from the A&P. "I've been in construction since 1966, and this is the worst I've seen it," he said. "I've been to two or three jobs today, and nobody has anything."

"People are more or less dejected, depressed," said Garner T. (Pete) Grover, a second-generation county commissioner. "Especially when they see someone next door or across from them closing up, the shopkeepers get depressed. And when the shopkeepers get depressed, the customers do, too."

Grover believes that if the media would declare "happy days are here again," the cloud would lift and people would spend. But they are not spending, and in some ways, said Grover, the recession is hurting Calvert County worse than the Great Depression of his youth, when at least the federal government was growing and the newcomers to Washington were spending their New Deal dollars in these parts.

While the new McDonald's, on Route 4 just outside of this sleepy county seat, is jammed, up in Dunkirk, at the county's northern end, DaVinci's Ristorante has had to change its image and its prices just to make ends meet.

For six weeks, DaVinci's offered two meals for the price of one, then a seven-days-a-week 20 percent discount, but neither promotion stemmed the $5,000 weekly losses, according to manager Harry Stone. So DaVinci's closed last week for remodeling, to reopen with a more modest menu. "The way the economy is today, you've got to change with the times or you don't exist," Stone said.

When Baltimore Gas & Electric Co. brought nuclear power and big bucks to the peninsula, the county boomed as never before. Spacious homes on large lots sprouted at the north end, filled with professionals who wanted country living within commuting distance of the city. With the poster promise of "Tranquil Living at Tidewater," Calvert grew by two-thirds in the 1970s. Of Maryland's 23 counties, only Howard grew faster.

The growth meant construction work without end, it seemed, for blue-collar residents. At the same time, "Keep Calvert Country" appeared on many bumper stickers. With the power plant providing more than 60 percent of the county budget and the building boom providing jobs, additional industry of any kind seemed almost superfluous.

"In a way," said Anne F. Whisman, a newcomer who edits a monthly magazine called Chesapeake Country Life, "the power plant made the county like the young fellow who inherits a million dollars and doesn't develop his own talents."

Whisman recently wrote to her delinquent advertisers: "We know times are tough, but we hope you'll put our name in the hat." A shoe store owner responded with a $70 check and a note: "The only excuse I can give you for being so late is we are using a barrel instead of a hat. Times sure are tough. We regret that we are not going to make it. Everyone has been good to us and supported us, but the county is in an economic disaster . . . "

Retail sales have plummeted in the county, which already had the lowest per capita sales tax payment in the state, and housing construction has ground to a virtual halt, with building permits down 44 percent last year.

Ed Howlin, whose construction company built more than 300 houses in the county, has laid off half his 40 workers and worries about the future. "If we ever do gear up, we've lost our high-caliber people," he said.

At Local 832, a predominantly black laborers union headquartered here, the outlook is likewise bleak. James W. Moreland, the business manager, has no good news for the men who stop by the second-floor office in a brick building next to the Courthouse. Nearly three-quarters of his 232 members are out of work.

"The guys are cutting wood or doing any little thing," he said. "Some have lost homes, had phones cut off. They're in the category of losing every damn thing they got. It's just critical. On Sunday, I had over 30 calls at my house, from people looking for jobs."

At the state unemployment office, a small brick building across from the Courthouse, a picture of President John F. Kennedy hangs on the wall, but there is none of Ronald Reagan. A broadside from the state says, "Budget cuts hurt all of us!!" It urges employes to call the White House to complain about cutbacks in employment and training programs.

Waiting to tell the unemployment caseworkers how hard they've looked for work were Ricky Gordy, 24, and Lester Harris, 47, both laid-off construction workers, like most of Calvert's unemployed.

"Before now, I had good years," said Harris, a laborer whose last job ended five months ago. "I built a house eight years ago. I've been on the edge of losing it. I'd give up my truck first, but then I wouldn't have any way to get to work. There ain't nothing I can do, just keep looking for work, that's all. I never thought it would be like this."

Gordy, a heavy equipment operator who hasn't worked in six months, spends his days "piddling around the house, playing with the dog, waiting for phone calls that never come, it seems like. I done filled out so many applications, I'm tired of it. The same routine, day after day, filling out applications--or they tell you just plain no, they aren't hiring."

When not looking for work, the unemployed of Calvert County can often be found at the watering holes along Route 4, the blacks at Burruss Club and the whites at Turner & Wells.

One afternoon last week, two dozen or so men and a handful of women were at Burruss'. Edward Stepney, 31, made a sweeping motion with his hand to indicate who was out of work. "We all are," he said.

A few miles up the road, at Turner & Wells, Donald Pitts, a 44-year-old unemployed carpenter sporting a red, white and blue cap with the letters "USA" emblazoned in front, was sipping soup in the backroom and bemoaning his month-long layoff. But he, at least, would not despair.

"Things will get better," declared Pitts. "We still live in the greatest country on earth."