[slededit]

Party line votes were historically more rare (under 50%). That has changed dramatically over the last few years and is a belleweather of the deep divisions in the country.

There were always divisive issues but there are also a lot of boring and mundane work. 90% party line votes are evidence the parties feel it's not politically safe to be seen agreeing with others on any issue regardless of how mundane. That isn't something you'll fix short of changing the electorate or getting rid of elections.

[dragonwriter]

In the period between the New Deal and now they were usually rarer because the New Deal Coalition broke the old party alignment and it took a long time before the parties each had a coherent ideological position again; that was a naturally unstable position.

It had little to do with boring, mundane work, it has to do with the fact that liberal Republicans and conservative Democrats—when neither of those meant centrist, they meant more to the left or right than the center of the other partt—used to exist, significantly, in both the electorate and elected office.

This was always an unstable condition, and the trigger for accelerating it's inevitable Denise was Johnson's civil rights position. But it still took a few decades after that for the realignment to complete.

[slededit]

Except that the current increase in partisan voting began in the 2000s. The 50s were closer in the mid 70% but for the period since partisan voting was under 50%