BILLINGS, Mont. (AP) — One of the largest coal-fired power plants in the western U.S. will close two of its four units in coming days as the Montana facility edges toward an eventual total shutdown.

Colstrip Units 1 and 2 — built in the 1970s when massive strip mines were being developed across Montana and Wyoming — will close by Jan. 5 or as soon as they run out of coal to burn, Talen Energy spokeswoman Taryne Williams said Thursday.

The plant employs about 300 people and is the main driver of the economy for the surrounding town of Colstrip, which has about 2,300 people. But it's been unable...