Regardless of your account of voter motivation, it seems as if both senators must cater to the same electorate. However, it should be noted that senators are frequently elected in different years. So one explanation for senator disagreement is that even though they legally represent the same state, they were selected by slightly different groups of voters. Pick one country other than the U.S. that you know something about. It has higher taxes, more socialized medicine, and more labor market regulation than the U.S., but weaker discrimination and anti-drug laws.
Canadian politicians adjust their platforms accordingly.
Suppose the U.S. switched to proportional representation in the House of Representatives. The Libertarian Party might get one or two. However, I doubt policy would change much. Name one policy of the federal government that you believe a majority of Americans oppose. Most Americans believe that we spend too much on foreign aid.