As environmental concerns drive power companies away from using coal, natural gas has emerged as the nation’s No. 1 power source. Plentiful and relatively inexpensive as a result of the nation’s fracking boom, it has been portrayed as a bridge to an era in which alternative energy would take primacy.

But technology and economics have carved a different, shorter pathway that has bypassed the broad need for some fossil-fuel plants. And that has put proponents of natural gas on the defensive.

Some utility companies have scrapped plans for new natural-gas plants in favor of wind and solar sources that have become cheaper and easier to install. Existing gas plants are being shut because their economics are no longer attractive. And regulators are increasingly challenging the plans of companies determined to move forward with new natural-gas plants.

“It’s a very different world that we’re arriving at very quickly,” said Robert McCullough, an energy consultant in Portland, Ore. “That wind farm can literally be put on a train and brought online within a year. It is moving so fast that even critics of the old path like myself have been taken by surprise.”