Who's the extremist?

Since 2000, several surveys have asked "overall, would you describe the views and polices of the Democratic/Republican party as too extreme, or as generally mainstream."  The results:

        R extr  R main  D extr  D main  R overall  D overall

08/2000      33 58 25 66          25     41

07/2004      40 55 34 62          15     28

09/2004      34 62 33 62          28     29

07/2010      39 56 43 54          17 11

09/2010      36   58 42 53          22     11

12/2012      53 43 37 57         -10 20

03/2013   48 47  42   53          -1     11

The "overall" is the percent seeing the party as mainstream minus the percentage seeing it as extreme, so the higher the score the better.  The perception of both parties as "too extreme" has increased, but the timing has been different.  The perception of Republicans was pretty steady through September 2010, but between 2010 and 2012 there was a big shift towards seeing them as extreme.   There was a shift towards seeing the Democrats as more extreme between 2000 and 2004, and another shift between 2004 and 2010.  

Over the whole period, the shift towards seeing the Democrats as extreme has been just about as large as the shift towards seeing the Republicans as extreme.  That seems surprising, given the absence of anything like the Tea Party movement among the Democrats.