The history here is well known to everyone interested in politics but worth summarizing. For most of the first 190 years of the country's operation, U.S. Senators would, in unusual circumstances, try to delay a vote on measures they opposed by "filibustering" -- talking without limit or using other stalling techniques. For most of those years, the Senate could cut off the filibuster and force a vote by imposing "cloture," which took a two-thirds majority of those voting (at most 67 of 100 Senators). In 1975, the Senate adopted a rules change to allow cloture with 60 votes, and those are the rules that still prevail.
The significant thing about filibusters through most of U.S. history is that they hardly ever happened. But since roughly the early Clinton years, the threat of filibuster has gone from exception to routine, for legislation and appointments alike, with the result that doing practically anything takes not 51 but 60 votes. So taken for granted is the change that the nation's leading paper can offhandedly say that 60 votes are "needed to pass their bill." In practice that's correct, but the aberrational nature of this change should not be overlooked. (The Washington Post's comparable story is more precise: "A bloc of 60 votes is the exact number required to choke off the filibuster, the Senate minority's primary source of power, and the GOP's best hope of defeating the bill.")
Again, this is a very well-explored issue in the academic literature
and much of the blog world. For blog and magazine discussions, see here, here, here, here, and here.
An authoritative academic treatment came from David Mayhew, of Yale, in
his 2002 James Madison lecture for the American Political Science
Association. It is available here in PDF and very much worth reading. Sample passage:
topic is supermajority rule in the U.S. Senate-- that is, the need to
win more than a simple majority of senators to pass laws. Great checker
and balancer though Madison was, this feature of American institutional
life would probably have surprised him and might have distressed him....
failure for bills not reaching the 60 mark. That is the current Senate
practice, and in my view it has aroused surprisingly little interest or
concern among the public or even in political science. It is treated as
matter- of-fact. One might ask: What ever happened to the value of
Everything I have mentioned here is familiar, including the fact that this newly-invented "check" was not part of the original check-and-balance constitutional design. But somehow it isn't familiar,
in the sense of being part of general understanding and mainstream
coverage of issues like the health reform bill. Talk shows analyze
exactly how the Administration can get to 60 votes; they don't discuss
where the 60-vote practice came from and what it has done to public
life. I have a gigantic article coming out soon in the Atlantic -- long
even by our standards! but interesting! -- which concerns America's
ability to address big public problems, compared in particular with
China's. The increasing dysfunction of public institutions, notably the
Senate, is a big part of this story.