I study the change in polarization in the US Senate from the period 1947–1966 to 1995–2014. I use a decomposition approach to quantify how much of the increase in polarization between those two periods can be explained by the representation relationship with all voters versus the representation relationship with copartisans. The decomposition also identifies the portion that arose from voters changing positions and how much arose from politicians changing how responsive they are to voters. I find that the representation relationship explains a majority of the increase in polarization. Furthermore, I find that Republicans have become more conservative because they have become more responsive to the positions of their copartisan base. Copartisan sub-constituencies matter for representation and are a source of increased polarization. The data also shows that Democrats have contributed to polarization because the overall electorate in their districts have become more socially liberal.