For much of his life, Randy Gray knew his hometown in northern Montana, Great Falls, as a stronghold for Democrats.

On the banks of the cascading Missouri River, a local smelter purified the copper wire that electrified America, produced by union laborers who were reliable Democrats. So too were the Catholics, committed to issues of social justice, who had followed early missionaries to the area. Even the farmers who tilled the fertile wheat fields of the northern Great Plains often supported the Montana-style Democrats who applied the principle of conservation to both tax dollars and nature.

Mr. Gray served three terms as a Democratic mayor, up through 2005. Barack Obama won the area in the 2008 presidential election, and Democrats cornered a vast majority of the county’s legislative seats. Jon Tester, the Democrat who has long represented the state in the U.S. Senate and farms in the same northern plains region, holds his election-night celebrations in Great Falls.

But the political landscape in Great Falls is not what it was when Mr. Tester first won his seat in 2006. With the old Democratic coalition frayed by economic upheaval, a flood of wealthy newcomers and increasing tension over delicate social issues, Republicans have captured one office after another in this part of the state. They now control every seat on the local county commission and all 12 legislative seats for the area — a political sweep that once seemed unimaginable.

“It’s like a meteor landing in your backyard,” Mr. Gray said.

The transition is part of a broader trend in Montana, where a proud tradition of ticket splitting is barely a memory. For Democrats nationally, the switch could not come at a worse time: Mr. Tester is now not only the last Democrat standing in statewide office in Montana, he may also be the last Democrat standing in the way of Republican control of the U.S. Senate.

Mr. Tester has defied the odds before, thanks to his broad appeal as a moderate, old-school Democrat and third-generation farmer who still cultivates wheat and peas when away from Washington. He continues to cast himself as a Democrat willing to reject his party’s leadership, declaring in one ad this year that he has “worked with Republicans” and “fought to stop President Biden from letting migrants stay in America.”

Thank you for your patience while we verify access. If you are in Reader mode please exit and log into your Times account, or subscribe for all of The Times.

Thank you for your patience while we verify access.

Already a subscriber? Log in.

Want all of The Times? Subscribe.