Silver to Loudoun and Density

Link to Silver Line to Loudoun article

This article makes the case the Loudoun will be come more Democratic because of high density housing near the metro stations. I accept the idea that people who live in high density housing lean more to the Democrats than people who live in low density housing.

What I wonder about is cause and effect. Is it that high density attract the Democrats, and so those people would vote the way they do regardless of where they live? Or does high density itself move people to the left?