BEMIDJI, Minn. — The planned route for this pipeline runs past small Northwoods towns populated with union laborers. It cuts across one Native American reservation, while steering around another. And it slices through the fragile coalition that has delivered Minnesota to Democrats in 11 consecutive presidential elections.

Influential Democrats have been among the loudest voices on both sides of a debate over whether to replace an aging pipeline, known as Line 3, which carries Canadian crude oil through the evergreen forests and pristine waters of northern Minnesota.

Politicians in the state’s big cities of Minneapolis and St. Paul, environmentalists, and many Native Americans, worried about the possibility of a spill and about climate change, have protested the replacement plan and held it up. Rural lawmakers and labor unions, seeing the potential for high-paying construction jobs, mostly supported it.

Over generations, the strength of Democrats in Minnesota was based on uniting the state’s city-dwellers and its rural residents in common political cause. But as Democrats prepare to vote in the Super Tuesday presidential primary, that alliance is frayed and in flux.