For centuries the sand dunes and marsh grasses of Assateague Island have protected the coasts of Virginia and Maryland from the Atlantic Ocean's fiercest storms.

Now the island, famous for the natural splendor of its beaches and shipwrecked ponies, faces another storm: this one political. The thunder is coming from two groups of wildlife lovers -- one on foot and the other in beach buggies.

And the lightning this time will be hurled by the Interior Department's controversial Secretary James G. Watt, whose call for greater use of national parks precipitated the fight here.

It is a simple question over how much of the 37-mile island -- now largely protected by Watt's department as a primitive beach and wildlife refuge -- should be open to Jeeps, trucks and other off-road vehicles. Currently they are restricted to 16 miles but a group of sportsmen, citing Watt's new park policies, has petitioned Interior to open another 10 1/2 miles of beach in the Chincoteague National Wildlife Refuge on the Virginia end of the island to their vehicles.

That stretch of beach, now called a "primitive zone," is beloved by hikers and birders, and coveted by surf fishermen who say it contains the two best fishing spots on the barrier island. Those locations, however, are remote and difficult for most people to reach on foot.

Assateague preservationists are outraged by the proposal, which they charge would destroy the beach and disrupt the fragile ecology of the gray, sandy shore. The fishing spots are also major resting and feeding stops for migratory birds, including hundreds of peregrine falcons, an endangered species.

Judith Colt Johnson, head of the Committee to Preserve Assateague Island, calls the proposal "the worst threat to Assateague Island in years."

Bill Shockley, president of the Assateague Sportfishermen Association, which supports opening the beach, says Johnson and the environmentalists who oppose his group have unfairly slandered four-wheelers as irresponsible joyriders. "We're nature lovers ourselves," says Shockley. "We love that island too. That's why we want access."

The debate beween the users and preservers has polarized not only local residents but park and wildlife officials from Virginia, Maryland and the federal agencies that have jurisdiction over parts of the island -- Interior and the National Park Service.

The Town Council in Chincoteague has voted unanimously to oppose increased access on the grounds that it would spoil the island -- the area's major tourist attraction -- and endanger its famed wild pony herd. Federal refuge workers privately say they have neither the staff nor the budget to supervise so many mobile people covering so much new ground.

Proponents of the new proposal have bypassed refuge managers and park superintendents, who historically have decided such use matters, and gone directly to Secretary Watt and his top officials. And they like what they're hearing.

"Now the atmosphere is so different, thanks to Secretary Watt and President Reagan," says Conrad Smith, president of United Mobile Sportfishermen Inc., which he says represents about one-third of East Coast off-road-vehicle owners, or 20,000 to 25,000 families.

"Prior to them there was an elitist group that for some reason or other supported this 'look but don't touch' philosophy. Save it for a million years. But save it for what? The way the world situation is today, everybody has nuclear capability; one big bang and everything's down the drain."

The Assateague proposal is being viewed by environmentalists and sportsmen alike as another test of Interior's new attitude toward public use of federal land. That attitude has already provoked the ire of conservationists across the country.

"It's a very emotional thing," says Craig Potter, an official with Interior's Fish and Wildlife Service. "There's a fear that Secretary Watt is going out and raping and ruining the land. That's wrong. He's trying to deal with delicate questions of resource use."

Potter says the debate over the Assateague proposal "gets to the heart of what's going on right now at Interior." He also says no decision has been made. But both supporters and opponents of the proposal are circulating a memo from the National Park Service which at least hints that Interior is about to reverse its position against beach buggies.

"We are particularly sensitive to the comments relating to what have been described as overly cautious or restrictive policies . . . . " reads the memo from National Park Service Director Russell E. Dickinson to G. Ray Arnett, Interior's assistant secretary for fish, wildlife and parks. "Particular attention will be paid to the seasonal usages of the various recreational groups . . . We should be able to provide better or more access to certain areas than are currently permitted."

The May 14 memo, which followed a meeting between Smith's four-wheel group and Reagan administration officials, bears the marginal comments "good" and "thank you" from Arnett.

Assateague has been fought over since 1939 when a storm severed it from Fenwick Island by cutting what became Ocean City Inlet. The ever-changing, narrow barrier island, which is bisected by the Maryland-Virginia border, includes primitive hiking areas, animal refuges and public swimming beaches for its more than 2 million annual visitors.

A herd of wild ponies, descendants of ponies thought to have survived a Spanish shipwreck in the 17th century, are the island's most famous residents. Each summer the Chincoteague Volunteer Fire Department rounds up the ponies to swim from Assateague to Chincoteague where they are put up for auction.

The effect of the vehicles on the pony herd has become part of the debate. The Assateague beach where beach vehicles are now allowed is separated from the Chincoteague refuge beach by a fence. The Chincoteague ponies, owned by the fire department, are separated by that fence from a herd owned by the National Park Service. The four-wheel sportsmen would like to see that fence either removed or gated so that they could make a through trip from either end of the island.

But the National Park herd suffers from infectious equine encephalitis, an incurable disease that was eradicated from the Chincoteague herd by destroying the animals.

"No gate system would work," says Mike Finley, the assistant superintendent of the National Park. "Any farmer will tell you that. There would be no way to keep the herds from mingling, and passing the disease." Infected animals may not be sold and any mingling would put an end to the annual pony auction that supports the volunteer firefighters.

No definitive scientific evidence exists on how much damage vehicles do to wildlife or the beach itself. Studies at Cape Cod National Seashore indicate that beach vehicles are not a major problem if they stay between the drift line at the toe of the dunes and the normal tide zone, as proposed at Chincoteague. Paul Godfrey, the chief Cape Cod researcher, emphasizes that his findings cannot be directly applied to southern beaches, which differ in everything from the composition of the sand to the number and types of beach organisms.

Because the Chincoteague wild beach is so narrow, there are places on the northern part of the refuge where there is no beach at all during spring and storm tides. For that reason, four-wheel vehicle users are seeking five crossovers where they can drive behind the dune line to avoid being trapped.

Up to 1.5 miles of new road would have to be cut through the heavy shrub growth behind the primary dune line, weakening, some say, the island's resistance to a major storm.

"Dune damage is a definite problem," says Finley. "It doesn't happen regularly but . . . . people drive over the dunes either out of ignorance or willfully, and that of course causes wave erosion and sand blowouts. Once the dune line is broken you can get the kind of overwash that cut us off from Ocean City."

Claudia Wilds probably knows Chincoteague's beaches and their birds better than anyone else, having surveyed shorebird nesting and migration there since 1974 for a Canadian-U.S. study. Wilds claims that migrant birds such as red knots and sanderlings, which generally rest between the dunes and the waterline, are displaced by vehicle traffic.

"This is a wildlife refuge; it's supposed to be managed for animals, not people, and certainly not for people in vehicles," says Wilds.

The four-wheel vehicle group is also seeking access to the six miles of wild beach at the north end of the island, which recently came under National Park Service control. "We are not actively considering that, although it comes up every time we meet with them," Finley said. "They want it during spring, winter and fall, but spring and fall are also the main hiking seasons, when the weather's good and the bugs aren't so bad. Our position is that they have 12 miles of beach already, and it's only fair that hikers and birders should have an area of their own."

"The idea of opening the north beach is absolutely preposterous. The area will be totally ruined," says Johnson, head of the Committee to Preserve Assateague Island.

"Let us prove ourselves," counters Shockley. "It might increase joyriding at the start; I'm not saying it won't, but after that it will mostly be limited to real hard-core fishermen. We're a very respectable organization; any organization has some fruits in it, including birdwatchers, and I'm sure we do, too, but I don't know of any."

The debate goes on and on. Refuge manager Dennis Holland has asked for comments on the proposal by next Thursday. So far the response has been more than 40 to 1 against beach vehicles.

But in the end, both sides agree, the Chincoteague question may turn on economics and political muscle. As Interior's Potter says, "In the background you always have political considerations . . . That's the way this country works."