Extremism is a powerful bludgeon in American politics. By claiming the other side is extreme, accusers can claim the high-ground of reason and popular support. Since the 2010 midterms, Democrats have liked to claim Republicans have moved far right. Implicit in this claim is that Democrats are where they were when Republicans began to move right. This claim has one problem– the exact opposite is true.
Since 1994, Republicans have moved slightly to the right but they are more or less where they were in 1994. Democrats, on the other hand, have moved to the far left since 1994. Using data from Pew Research, we can see where the median Republican was in 1994 compares to where they are today. While there was a slight rightward shift for the median Republican, the shift for the median Democrat is clearly multitudes more.
We can take the same Pew Data and limit our population to only look at politically active Republicans and Democrats. In this data set, the movement for the median Republican is almost nonexistent. While the median Democrat appears to be slightly less (because they started farther left) they still clearly changed by a larger magnitude.
To put a finer point on it, here is the shifting year by year.