It's their duty to represent everyone in their constituency, not just the ones that are members of their political party. I'm not sure that's true. In that cas

It's their duty to represent everyone in their constituency, not just the ones that are members of their political party. I'm not sure that's true. In that case, there would be a moral argument against voting for third parties, since they definitely don't represent the views of a majority of people. And there'd be a moral argument against voting Republican in Manhattan, or Democrat in Utah.