ADA Shows that America is more polarized, and who is causing it

Americans for Democratic Action, a liberal advocacy group, rates House and Senate members for how well they vote for liberal interests.  All perfectly fine.  A recent study and explanation can be found here.  The graphs (also below) clearly shows more polarization.

So, who is causing this polarization?  Both the Democrats (blue) and Republicans (red) are voting more "in lockstep", as shown by the narrowing bands of color.

However, liberal papers like the New York Times constantly complain of the Republicans moving to the "far right", to the "extreme".  Are they?  From the picture, it appears that they may be moving slightly to the right (down).  However, in comparison, what is immensely clear is that the Democrats are moving strongly to the left (up).

So, according to a liberal advocacy group, and their data, it is not the Republicans, but the Democrats who are moving to the extremes.