Republicans had elevated a Democratic candidate who promised not to clear the way for an independent challenger in a conservative state.

Montana voters will select nominees in Tuesday’s state primary to replace two prominent Republican incumbents: U.S. Sen. Steve Daines and U.S. Rep. Ryan Zinke.

Montana has not been at the top of the list of seats Democrats hope to flip to regain control of either body, but the retirements do creak open the door for a candidate to…