Here’s some interesting quantitative analysis from Alan Abramowitz, illustrating a point that seems qualitatively true. In the 1970s, voting for both the House and Senate was only very weakly correlated with presidential voting. In the 80s, the correlation got stronger. In the 1990s, the correlation got stronger. And in the 2000s, it’s got even stronger.
I’d say it’s largely a good thing that we now have more ideologically coherent parties and a more rigorously consistent political system. But political institutions need to adapt to that reality.