US political polarization

A good article in the NYTimes describes the partisan and often bitter nature of the 60-39 party line vote this morning on health care.  Offerred explanations are interesting, but conflicting:
1.  Party line voting comes because  the parties have each become ideologically narrower.
2. Party line voting comes from practicality because politicians are afraid to vote outside the party position.
3. Party line voting is associated with the increase in lobbying moneys spent.

I don't really have an opinion on this.   Clearly the capacity to vote outside the party line(as was done in 1965 on vote creating Medicare when 13 Republican Senators voted for, and 7 democrats voted against) is disappearing.  And if the primary reason is actually to defeat the other party in an upcoming election, it seems we've lost the sense of the greater interest of the people....another version of the "end justifies the means."

It makes me think again about the 60 vote requirement in the Senate to bring about debate....which in turn makes me want to read Lani Guinier's The Tyranny of the Majority  I just ordered it used on Amazon.