What I am having trouble reconciling is how people can have this belief (that individual offices should be filled by people who are committed to enacting the will of their constituents) with the vote-blue-no-matter-who approach.

If there is no nationwide platform for the parties that all candidates are expected to uphold, and the expectation is that the people in office are supposed to carry out the will of their constituents, voting along party lines seems entirely meaningless.

(I am not seeking to understand why this arrangement makes sense overall. I know it doesn't and that the US electoral system even as designed originally was meant to make it impossible to represent local desires in favor of prioritizing preservation of the union. Nothing has changed. My question was meant to understand the perspective of the people who believe these two things can be true at the same time.)