This Pew chart provides nice evidence of a phenomenon I been looking into.
The chart shows how voters perceive their own ideological postion and the ideological position of various Democratic and Republican candidates. The top graph shows that the aggregate ideological placement of all voters is somewhat right of center. Furthermore, the positions of the Republican candidates is much closer to the position of the average voter than are the positions of the Democratic candidates.
The next two graphs show this same placement for Republican and Democratic voters. Notice that there isn't much difference between how Republican and Democratic voters perceive the Republican candidates. Now look at the placement of the Democratic candidates. The Republicans see Clinton and Obama as significantly more liberal than do Democrats. This difference in perception--both Democrats and Republicans agree on the position of Republican candidates, but differ on the position of the Democrats--is known as left-shift. It's reasonable to assume that partisans will see opposite party candidates as more extreme than they really are and their own candidates as more moderate, but this only seems to happen with Republicans. In other words, Democrats and Republicans agree that the Republican candidates are a pretty conservative bunch, but Republicans see the Democratic candidates as much more liberal than Democrats do.