For much of the American shale boom of the last two decades, natural gas producers found that the more they pumped, the more demand grew, as cheaper gas displaced coal.

But here in Pennsylvania — home to one of the largest U.S. gas deposits and a critical prize in the presidential election — that is no longer the case.

The state’s hilly southwest, where gas extends beneath homes and river valleys, is so awash in the fuel that prices have cratered, drilling has slowed and thousands of jobs have disappeared.

While Pennsylvania and energy policy are contested battlegrounds between Vice President Kamala Harris and former President Donald J. Trump, the bleak reality of the gas business in the state has been obscured by sparring over horizontal drilling and hydraulic fracturing, commonly known as fracking.

Mr. Trump supports fracking, and Ms. Harris, who called for banning fracking in 2019, recently said she no longer opposes the practice.

For many voters in the Appalachian region of Pennsylvania, the key question is not how to produce more natural gas, but how to build the infrastructure necessary to get it to places like New England and the Gulf Coast where it is likely to fetch higher prices.