Keith Poole is a major figure among political scientists, having gathered data on every significant congressional vote since time immemorial and analyzed those voting patterns. I just realized that he has put a ton of data and analysis online. It's fasacinating stuff. Here are his data on polarization (note that party polarization is at its highest level since reconstruction). Here is his paper finding that gerrymandering does not cause polarization, and here is his paper finding that members of Congress adopt a consistent ideological position and maintain it over time.
But what spurred me to write this post are his data on the ideological mapping of Obama, Clinton, McCain, and Bush. The analysis shows very little distance between Obama and Clinton (both slightly to the left of the middle of their party), but what's really interesting to me is the gap between Bush and McCain, and in particular how far to the right Bush is. Also interesting is how McCain's voting party moved him to the left of his party as the parties became more polarized. Of course, a single left-right axis flattens out a fair amount of complexity, but they also have two-axis measures that capture more, and in any event their data still capture a fair amount. (If you want more on the reliability, validity, and signficance of Poole's data, there's tons on that. His numbers have been used for so long, by so many political scientists, that just about every aspect of them has been considered exhaustively.)