In the environmental wars between diggers and conservationists, Rep. Richard B. Cheney Jr. (R-Wyo.) is usually found on the side of the diggers.

He fervently supports oil and coal development in his state. He's an admirer of Interior Secretary James G. Watt's dig-it-up-now approach to energy resources. He's the despair of environmentalists, who calculate he voted their way only 12 percent of the time--compared with a House average of 48 percent--during his first term.

Yet today Cheney and other conservative western Republicans are unlikely rebels in the touchiest environmental fight of the year, the clash over mineral leasing in the nation's designated wilderness areas. They're opposed to the oil and gas companies that are lining up with applications to explore in the supposedly sacrosanct wildernesses before a deadline cuts them off at the pass.

To explain his position, Cheney points to a map of Wyoming where the pristine Washakie Wilderness Area is the target of about 150 mineral lease applications. Roughnecks in the oil fields like to hunt and fish and picnic there, he says, adding: "We want the northwest corner of the state left alone. Ninety percent of the state is up for development and we want the other 10 percent as it is."

Similar skirmishes dot the western wildernesses from Montana to New Mexico and they all come to a head this year in Congress, which may decide whether all of the areas should be put off bounds permanently to developers. Under strong pressure from home, several western Republicans who may hold the key votes are preparing to side with the conservationists.

The issue parted some old friends recently when Sen. Malcolm Wallop (R-Wyo.) bluntly warned the pro-development Rocky Mountain Oil and Gas Association that public opinion was generally against some of its designs on the Washakie. "You cannot ignore this rising tide," he said in a speech. "Some of the most ardent anti-wilderness people I have ever known are very protective of the wilderness we do have." In a year when Big Oil is already under attack, the industry would get its fingers burned if it tackled the wilderness, he said.

The wilderness confrontation has reached an emotional turning point because of a deadline in the law and because of uncertainty about Watt's attitude. The original Wilderness Act permitted the government to issue exploratory mineral leases in those areas until midnight Dec. 31, 1983. It's been the policy of past administrations not to issue leases, on grounds that Congress really intended the areas to remain untouched.

With the deadline less than two years away, however, potential developers have been eager to obtain exploratory rights. By one count, they are currently applying for between 950 and 1,000 leases that would encompass about 3 million of the 25 million acres of designated wilderness in 48 states, excluding Alaska and Hawaii.

Moreover, it has recently been disclosed that over the past 10 years low-level Interior officials actually have issued at least 45 leases in several western states, including Wyoming, Colorado and New Mexico. The local furor that accompanied those revelations caught members of Congress by surprise and set them howling at Watt's door.

When Rep. Manuel Lujan Jr. (R-N.M.) learned from a newspaper report last fall that three leases had been issued in the El Capitan Wilderness in his state, he was so provoked that he introduced legislation that would have immediately barred all leasing in any wilderness area. That led to a confrontation with Watt, who agreed to a moratorium prohibiting any leasing until this June. Lujan, usually an administration supporter, says now that it was merely his way of calling attention to how loose and easy the leasing policy is.

"You can walk into any wilderness and start prospecting even before you get the lease, and once you prove there's a significant amount of resources in there, the BLM Bureau of Land Management has no authority not to issue a lease," Lujan said. "My main concern is that if there is to be leasing it should be done at a high enough level to be a sensitive issue."

Watt's Interior Department has as yet to spell out a wilderness leasing policy, although its fervor for encouraging energy development of other public lands is well known. Interior officials recently confirmed that in at least one instance Watt went on record favoring extension of the December, 1983, deadline, thus giving oil and gas interests more time to put in their bids. He has also told members of Congress that the present law gives him no choice but to issue leases if certain criteria are met, and that if Congress feels differently it must rewrite the law.

But Watt also is feeling the political heat from anxious congressional Republicans who usually are friendly. The secretary recently extended the leasing moratorium until after the elections this year. Conservationists called that a political ploy to ease the pressures on western Republican officeholders and on himself.

A number of congressmen, both liberal and conservative, intend to press anti-leasing amendments anyway. The result may be to banish leases from the wildernesses forever. "If I went into the House Interior committee and laid down a bill tomorrow to stop all leasing, it would pass in a snap," said Cheney, "and it would have a lot of support on the House floor."

One such blanket ban already has been introduced by Rep. Phillip Burton (D-Calif.). A compromise favored by Cheney and others would ban all leasing immediately and call for a government inventory to discover just what resources exist in the wilderness areas. They would remain immune to private exploration and drilling unless both the president and at least one chamber of Congress agreed that a national emergency necessitated recovering the minerals. Cheney said he has argued this position with the administration, but does not know if it will prove acceptable.

One of the mysteries is that no one knows whether the wildernesses really contain much oil and gas. Professional conservationist organizations say there is little, citing one of their studies that claims only about 1.4 percent of the nation's provable onshore oil potential is located in wilderness areas. Critics scoff at that estimate, insisting that the real quantity is unknown and that surveys either by the government or private developers are necessary.

Conservationists contend that even exploratory leasing endangers the wildernesses, because it allows developers to build roads and bring in trucks and heavy equipment. The land would remain scarred and wildlife would be sent scurrying to other, less protected areas, they argue.

"If they run all of the grizzly bears out of one county, they'll never come back," insists the Sierra Club's Tim Mahoney. "There are many small wildernesses, like in Virginia and Vermont, where if you build one road in and sink one well, it's gone."

The stage for this year's encounter will be the House Interior Committee, which is usually neatly balanced between members who favor resource development on public lands and those who oppose it. The opposition this time by usually acquiescent conservative westerners may tip the scale. Pressure from back home, those members report, is almost overwhelmingly against opening up the untouched regions. Several said they hoped the White House will intervene to spare them a direct confrontation with Watt.

The local pressures, the westerners say, become intense every time there is a report of a new lease application and it builds up fast to a point rarely experienced. Sen. Wallop described the change this way:

"There's always been pro- and anti-wilderness sentiment in Wyoming. And there are a lot of people who are against creating any more wildernesses, but even they are among the most rabid in saying that we should keep the wildernesses we do have. The sentiment to keep the wilderness covers the whole spectrum now."