It's easy to think of the Democratic and Republican parties as mirror images of each other or assume that trends influencing one party are influencing the other. But that's wrong in a whole number of ways.
This chart from Robert Boatright looking at congressional primaries shows one way that symmetry has been failing lately — ideologically motivated primary challenges have become much more common on the Republican side, while there's been no change whatsoever on the Democratic side.
An interesting question is whether this will change in the future.
On its face, the Bernie Sanders campaign appears to suggest the existence of substantial untapped demand for left-wing ideological primary challenges in the Democratic Party. Sanders has shown there's a considerable amount of money out there to back left-wing candidates. And the fact that his supporters are so disproportionately young suggests that in the future there will be more and more grassroots enthusiasm for left-wing candidates.
Sanders has not, thus far, shown a ton of interest in down-ballot issues, choosing instead to continue raising tens of millions of dollars to spend on TV ads in New York, Pennsylvania, New Jersey, and California. But if he and the coalition behind him turn in that direction, they could bring balance to a political system in which ideology currently plays a much bigger role in the GOP than it does on the Democratic side.