GORDONVILLE, PA. -- Andrew Kinsinger, a hearty man of florid features, is an unofficial lay leader of the Amish, a retiring and austere sect that has tilled land in Pennsylvania's Lancaster County for more than two centuries.

It is fertile, gently sloping land, through which the state says it may want to build a limited-access, four-lane highway. Kinsinger was in quiet conversation on his front porch, discussing what is to the Amish a troubling prospect.

From time to time, Kinsinger, 66, had to break off the discussion, unwilling to speak above the shrill rush of traffic on the narrow road a few yards from his home. From beyond a low ridge not far off came the steady rumble of trucks grinding along U.S. 30 through the southern reaches of Amish country in Lancaster County.

Relentlessly, vehicular traffic intrudes upon and defines the simple life of the plain-clad Amish, a sect that eschews auto ownership. In horse-drawn buggies, they take to roadways congested by automobiles and buses, many filled with tourists fascinated by the people popularized, unwillingly, in the motion picture ''Witness.''

To ease the chronically dense traffic on U.S. 30 and state Rte. 23, a two-lane road to the north, state transportation officials have proposed the four-lane highway that could cost $100 million to $150 million. The objective is broadly endorsed, even by the Amish, albeit quietly.

''We need a road, but we don't want it to go through prime farmland,'' said Levi Esh Jr., a young farmer.

Six principal routes for the highway have been designed. Three of them are between routes 23 and 30, the heart of the oldest Amish settlement in the country.

A route through the heartland would likely have multiple consequences: It could sever many of the compact Amish farms, divide the intricate network of Amish church districts, and close some of the narrow backroads vital to rural commerce.

Opposition among Lancaster County officials ultimately may force the thoroughfare away from the Amish heartland.

The officials' reservations deepened after a meeting late last month convened by the Pennsylvania Department of Transportation to describe the proposed project. More than 1,000 Amish attended in what experts on Amish life said was an extraordinary expression of concern.

''It was a dramatic occurrence, but it was all within their tradition. They gave a strong message just by being there,'' said John A. Hostetler, a leading authority on the Amish. The Amish refuse welfare payments, spurn military service, operate their own one-room schools and live without many trappings of modernity. They are permitted to vote but decline elected office.

''Our custom is to live day to day. But you cannot help but be concerned'' about the consequences of the highway, said Kinsinger, the chairman of the Old Order Amish national steering committee, a low-key body that addresses internal disputes. Old Order Amish is the sect's most conservative branch; 14,000 Old Order members and their children live in Lancaster County.

''Some of us would be harmed'' by a highway, Kinsinger said. ''But it would not mean the end of the Amish in Lancaster County.''

State transportation spokesmen are emphatic in saying residents' opinions will be vital in fixing the course of the highway.

''It's in our best interests to keep an open mind. We have no preconceived notions. ... no rams projects down anyone's throats anymore,'' said John L. Rautzahn, an engineer working on the project. ''Every route takes quite a bit of farmland. It's a question of whose farmland it is.''

Philip D. Miller, a colleague of Rautzahn's, said the state would be remiss not to consider routes through Amish country. ''You have to analyze any option you have and weigh it against the others,'' he said. ''It's standard engineering practice.''

Miller also said ''it is not inconceivable'' that technical and environmental studies will show that the only practical routes are through the Amish country. Two of the six proposed routes are so circuitous that they seem ill-suited. The most elaborate proposal, a 30-mile project, may be too expensive.

The state will disclose its preference at a meeting in mid-December, when the audience will be surveyed for its preference. The route is to be designated late next year. Construction is not likely before 1991.

The Amish assuredly would not turn defiant if the highway goes through their farmland. ''They'd just passively accept it,'' said Hostetler. Their ''ethic says if they take it away from you, let them have it. You don't fight in a war or in an argument.''

''We're not the people to step in there and block the roads and say this is just not going through these farms,'' Kinsinger said. ''That isn't our nature. That isn't our way of doing things.''

However, Amish sympathizers are more vehement in their opposition. Among them is Richard Armstrong, an airline pilot who wrote tracts about the proposed highway and distributed them in Amish church districts, prompting the large attendance at last month's meeting.

''You put a road in here and you're paving over a culture. And {the Amish} won't have any choice but to leave,'' said Armstrong, who has lived in Amish country since 1979.

''There are more people with cars who stand to benefit than there are Amish,'' he said. ''I don't trust stuffy bureaucrats who say, 'We're not going to shove roads down their throats.''

Still, the prospect of a four-lane highway seems remote and incongruous to visitors to this fertile land -- a succession of compact dairy farms, windmills and domed silos. Well-tended fields of corn and alfalfa form long, neat rectangles.

The land yields abundance readily. Its gentle slopes would yield easily to a four-lane highway.

However, a consensus is emerging among county officials that the highway must skirt Amish country. The transportation department would be unwilling to build the roadway without county support.

County Commissioner James Huber, who is running for reelection, said last week that he opposes a highway through the Amish area. Such a project, he said, could ''kill the goose that laid the golden egg here in Lancaster County,'' the thriving tourist trade loosely based on Amish culture.

John R. Ahlfeld -- planning director in Lancaster County, where the population is growing each year by 4,000 -- said: ''It has become obvious that some of the {prospective routes} are just not going to be acceptable because of their intrusion into prime farmland.''

''The challenge is to accommodate the growth we get as efficiently as we can,'' he said. ''People are afraid that one day they'll drive to work and won't pass any farmland.''

Some Amish said they believe a highway would stimulate growth. ''The more businesses that come in, the more it's going to be crowded, the more it's going to be inconvenient. It's already crowded and it's going to be more crowded,'' said one Amish farmer.

The proposed highway is only one of the challenges confronting the Amish in Lancaster County. ''In a sense, they're at the mercy of industry and highways and tourism,'' Hostetler said. ''There's not much they can do about it.''

Over the generations, the Amish farms have been divided repeatedly and passed from father to sons. Many farms are now 40 to 50 acres, too small to be split again. So some Amish families have sought land elsewhere. To supplement incomes, many Amish have opened small businesses at their farms.

Increasingly, the Amish are participating, if only marginally, in the well-developed tourist trade. Some sell quilts and produce from roadside stands. Many young Amish women work in the restaurants and hotels.

''There is a slow {process of} getting used to the world's ways on the outside,'' Hostetler said. ''More Amish get jobs working in industry and small shops, leaving home to work on a daily basis for industry in the area, {creating} a form of dependence on the outside.''

However, the prospect of the highway could galvanize the Amish.