The nation's coal miners once had the thundering voice of John L. Lewis to speak for them. The lion-like Lewis fiercely battled presidents, Congress, coal companies and the courts to win decent wages, better mine safety and adequate pension funds.

Unfortunately for the miners, the Gideon's trumpet of John L. Lewis has become a tin whistle when it's blown by his tremulous successor, Arnold Miller. The once mighty United Mine Workers union seems to be disintegrating at the top, although it still has strength among the ranks in the coal fields of Appalachia.

The union was shaken when the man Lewis chose to be his successor, Tony Boyle, was found guilty of masterminding the brutal murder of union rival Joseph "Jock" Yablonski and his family. Miller came up from the coal pits to lead the battle for reforms. He was hailed as a white knight when he was elected president on a reform ticket.

But the current coal crisis had demonstrated his failure as a labor leader. His own union board voted down a tentative agreement that he reached with industry leaders. It took government pressure on the local companies to get him a better offer. But he failed by a resounding 2-to-1 ratio to persuade the miners to accept it. There is now a strong movement afoot to oust him.

We sent our associate Hal Bernton to the coal towns of Kentucky to find out what is happening to the union at the grassroots level. He found that union solidarity and a fighting spirit still prevail along the catwalks and back trails of coal country. But loyalty to the national leadership of Arnold Miller is fast diminishing.

In the eastern Kentucky mountains of McMcreary County, for example, the national union faces a crucial test of its ability to expand. For 18 months, a small band of miners has waged a savage fight for union recognition, with only moderate support from union headquarters in Washington.

In the bitter Kentucky winter, Bernton found a tiny union outpost set up in a plywood shack overlooking the Justus mine, which is owned by the Blue Diamond Coal Company. Since late 1976, a small band of volunteers has been picketing the mine, trying to obtain union recognition without success.

Each day, a crew of six reports for picket duty on the pine-wooded outskirts of the company property. A potbellied stove helps stave off the cold in the shack for off-duty pickets; a game of penny-ante poker helps them stave off boredom during the 12-hour shift.

Things were quiet during Bernton's visit, but in previous stages of the strike, there was sniper fire, state police action and jailings. Last fall, the pickets exchanged shots with the security guards who patrol the mine for Blue Diamond. Trenches were dug, sandbag walls erected. At least five have been wounded in the hostilities.

Blue Diamond has a record of obduracy to workers' safety. The company's Scottia works in Letcher County was the site of a 1976 disaster: Two explosions took the lives of 26 men. Because of the company's poor safety performance, the government has barred Blue Diamond from any federal contract awards. U.S. mine inspectors have reported the company as "very slow to react to hazard" to the safety of its workmen.

In the strike that has shut down the mine, the union has zeroed in on the safety issue. Pickets charged that inexperience workers were assigned hazardous underground jobs they were unqualified to do. They tell of poor ventilation, with heavy accumulations of coal dust, which results in black-lung disease; of the lingering presence of explosive methane gas in working shafts.

One striker told Bernton: "Young miners have opened their eyes. They've seen men come out of the mines with fingers cut off and head mashed flatter than a pancake."

Blue Diamond's founder, Alexander Bonneyman, was credited with being a benevoletnt owner when he came to the area from Knoxville in 1915. But his son Gordon has taken over the mine. He is a hard-nosed, anti-union boss who resist pressure from local government officials and his own Catholic church.

Jimmy Greene, McCreary County judge and chief executive, bitterly commented: "Blue Diamond had no contact with community leaders. They are stricly an absentee landlord. If there had been a closer relationship with the county officials, perhaps the strike wouldn't have stalemated."

Even without a strike, McCreary, the poorest county in Kentucky, has profited little from the abundant coal. "The county is just living off the federal government," the editor of the local paper observes. From Washington, the local government gets an annual $250,000 dole. Another $72,000 is derived from a state tax on coal that leaves the county.

Last year, Kentucky collected $112 million in coal severance taxes, but of the total, only $18 million found its way back to the 41 coal counties that produced the revenues.

McCreary's roads are typical of coal country, atrociously rutted by heavy truck traffic and left unrepaired because of lack of money. Schools must shut down with every snowstorm because there's no equipment to clear the highways.

The coal companies aren't paying their share of property taxes, thus impoverishing a region already despairingly poor. In Harland County, University of Kentucky studies show that out-of-state mining firms own 48 percent of the land, but pay only 20 percent of the taxes because of undervalued assessments.

In that bleak, harsh landscape, a new breed of miners is emerging. The sons of the men who struck when John L. Lewis told them to strike and went back when he directed still believe in the union. They know that under President Carter's energy program, coal will play an important role, and prosperity may lie ahead.

But they also know that Arnold Miller has paid only one visit to the front line of the little war at Blue Diamond's Justus mine and then only at the urging of Harry Patrick, a former national treasurer.