The last Congress continued the trend of the two parties moving apart, with Republicans moving right faster than Democrats moved left.
The Senate is polarized entirely, such that even the most conservative Democrat is noticeably more liberal than the most liberal Republican.
New data suggests that most competitive Senate races give voters a choice between two candidates who are quite far apart ideologically.
Contrary to popular belief, Democrats are actually better at party discipline than Republicans. And both parties are extremely unified, to an unprecedented degree.