DECKER, Mont. — The Trump administration is wading into one of the oldest and most contentious debates in the West by encouraging more coal mining on lands owned by the federal government. It is part of an aggressive push to both invigorate the struggling American coal industry and more broadly exploit commercial opportunities on public lands.

The intervention has roiled conservationists and many Democrats, exposing deep divisions about how best to manage the 643 million acres of federally owned land — most of which is in the West — an area more than six times the size of California. Not since the so-called Sagebrush Rebellion during the Reagan administration have companies and individuals with economic interests in the lands, mining companies among them, held such a strong upper hand.

Clouds of dust blew across the horizon one recent summer evening as a crane taller than the Statue of Liberty ripped apart walls of a canyon dug deep into the public lands here in the Powder River Basin, the nation’s most productive coal mining region. The mine pushes right up against a reservoir, exposing the kind of conflicts and concerns the new approach has sparked.

“If we don’t have good water, we can’t do anything,” said Art Hayes, a cattle rancher who worries that more mining would foul a supply that generations of ranchers have relied upon.