It hardly seemed possible to the plaid-shirted foresters who make their home in the solitude of the rugged wilderness deep in central Idaho.

As the men braced themselves a dark olive twin-rotor Jolly Green Giant helicopter touched down in front of them, its downdraft ripping shingles off their log cabin's roof. "Well, look at that," said Bud Hamilton, a veteran U.S. Forest Service backwoodsman. "Ain't that a sight?"

The huge helicopter was carrying government agents and young military communications men to primitive airstrips in the deep canyons through which the Middle Fork tributary of the Salmon River flows. They were setting up camp in preparation for President Carter's 70-mile-long raft trip down the river this week. On the river an armada of 20-foot-long black rubber rafts bobbed about, filled with Secret Service men and FBI agents.

Clad in camouflaged hunting caps, blue jeans and Army field jackets, the feds jumped out at various spots to set up observation posts and communication points. Antennas bristled along the river among the ponderosa pine and men with short hair and silver sunglasses peered at vacationers from behind strategic boulders.

"I hope these guys are going to be okay out there," said Hamilton. "You know only one of those guys said he had any real outdoor experience. He told us he had been a Boy Scott."

Not only for the backwoodsmen are these uneasy days in Idaho, that hatchet-shaped realm of ranchers and potato farmers living a way of life threatened mainly by the federal government and refugees from California.

As farmer Sheldon Hawkes said, "You get the feeling that something bad is going to happen. I don't mean losing a good crop or 100 head. I mean outsiders are beginning to discover us and I don't care if it's the president or some out-of-state real estate investor, it don't look good."

The president arrives tomorrow and few events could so neatly capture Idaho's dilemma. He arrives in search of solitude. He brings with him - reluctantly - its antithesis: hundreds of reporters, television technicians, Secret Service, the nuclear war "button" military communications experts, sophisticated radio communications apparatus, advance men and jets.

While he rafts down the Middle Fork of the Salmon River, amid one of the nation's largest wilderness areas, helicopters will be ferrying video-tape accounts for transmission via the networks. While he sleeps on the river's banks, the next pool of reporters will be gearing up to follow him in the morning.

After years of a net loss of populations, Idaho has joined the fast-growth states. It now has 850,000 people, but 1.5 million are foreseen by the year 2000.

As soiologist Robert Banks says, "Idaho is one of the few states where it is possible to come without a job and know that you can still live the good life. There is a fragile beauty here. It is not a myth."

"Idaho," reads a T-shirt sold at the Boise airport, "is what America was."

More than 60 percent of the state is federally owned, which has long angered timber, mining and ranching interest. Protected forest lands, such as the Idaho Primitive Area that Carter will raft through, should be turned over to the state and eventually opened to industry, these interests believe.

Even with the traveling White House, Carter may still find an isolation never before matched by a modern president.

There are no roads along the 104-mile North Fork of the Salmon River. There are no Winnebag campers, toilets (one-hole pits are used) or electrical outlets. There are just a million and a half acres populated by deer, bighorn, sheep, salmon and, in some areas, rattlesnakes.

The campsites the president's party of 20 will use are no more than sandbars that can accomodate no more than 30 people at a time, according to Wally Shiverdecker. "Ther military will be involved in some of the transportation."

"They were such nice men," said 73-year-old Audrey Hill, who lives in a cabin with a large, wood-fired stove, of the advance men. "I told them there'd always be some peanut butter cookies and a pot of hot coffee here if the president wants to visit."

Her husband, Eli Hill, 62, said he had chatted with the Secret Service about the campers and other rafter who may be in the area when Carter is.

"They wanted to put an agent at every campsite," an awed Hill said. "I said, 'That's fine.' I asked them, 'What about the backpackers? There are lots of them wandering through the mountain and what are you going to do about them?"

"The Secret Service man didn't look too happy. 'We don't want any people popping up suddenly behind rocks,' he said."

The networks aren't very happy either. Jim Lee, White House field producer for NBC News, said that initially NBC was planning on having TV crews hike with their equipment along the secluded trails to find vantage points for the cameras.

They made a careful study of geologic maps to find landing areas for aircraft to retrieve the videotapes.

Utlimately, the White House, which has been closely guarding the president's privacy, agreed to a "protective pool" of reporters and technicians that will raft 10 to 15 minutes behind the president and camp nearby.

The tapes will be carried out by helicopter to Boise, 180 miles away, where the rest of the press will be headquarter in a motel.

Lee and most reporters who will be covering the trip readily concede the likelihood that there will be no news. "But there's the president on vacation," Lee said. "And the protection thing. What happens if something happens to him? What happens if there's a world crisis and he's drawn into it? We want to be in a position to cover it."

The president is coming to a peculiar state. Politically, it is among the most conservative. One of its Republican congressmen, cherry grower Steven D. Symms, said last week that the country should be thankful Carter is coming because "for three days he won't be able to do any damage to the rest of the nation."

Perhaps a quarter of Idaho's residents are Mormons. They live in the Republican southern half of the state. The northern half, with its heavily blue-collar mining and timber industries, is mostly Democratic. Thus, while Idaho sends a Steven Symms to Congress, it can also send Sen. Frank Church, a Democrat.

Carter's popularity is, as business Jack Holt says, at "rock bottom." The state voted almost 2 to 1 for Gerald R. Ford in 1976.

Idaho has a Democratic governor, John V. Evans, who succeeded the phenomenally popular Cecil D. Andrus - who will accompany Carter here - when Andrus became secretary of the interior. Yet, even though Evans is running for election this November, his staff is debating how much socializing he should do with the president.

The capital city, Boise, was named for its trees by French trappers. A corporate headquarters town with virtually no industry, its population is about 100,000. But it has a 24 percent growth rate, and new ranks of subdivisions are eating into the sagebrush rangelands. Unemployment is 2.4 percent.

Lines of sleek Lear jets, both private and corporate, sit at resorts such as Sun Valley and McCall while their passengers spend a weekend shopping for land - pretty much, resentful Idahoans think, like they would at their Los Angeles supermarket.

Idaho has become fiercely chauvinistic in the last few years of growth. Bumper stickers reading "Don't Californicate Idaho" are understandable. More than 30 percent of Idaho's new immigrants come from there.

But so strong is the feeling here that Idaho is about to be deluged with change that a sticker reading "Don't Oregonize Idaho" is being seen, and, as is widely known west of the Rockies, Oregon has long been seen as the New Jerusalem.

But Idaho too, it appears, has become a retreat for thousands of Americans disenchanted with big-city life. Many small Idaho towns now boast a lawyer or doctor who left California for a more simple way of life, which includes backpacking, skiing and white-water rafting.

Balladeer Carole King, who lives on a ranch near Boise, gave a concert last weekend near Sun Valley. After the audience applauded songs from her new album, "Welcome Home," its lyrics focusing on Idaho life, she told them they had a special duty. "If you love Idaho," she said, "keep it the best-kept secret in America."