- There is nothing about the filibuster in the Constitution, which does say that the Senate (like the House) shall set its own rules and which specifies a few conditions (treaties, impeachment, veto override, etc) requiring more than a simple majority vote;
- Through the 19th century, Senate rules and practice evolved to allow unlimited debate that could block votes on certain bills -- that is, a filibuster;
- Nearly a century ago, in 1917 during the Woodrow Wilson administration, the filibuster rules most people associate with that word were set. Senators who opposed a measure could talk as long as they wanted, to prevent it coming to a vote -- unless two thirds of their colleagues voted to cut them off, through "cloture". But it had to be "real" debate, or at least oratory, with Senators holding the floor and talking till they dropped;
BUT THEN, in 1975, the rules changed, in a way that eventually gave us our current filibuster-everything-without-really-filibustering environment. A reader takes up the story from here:
Many of your readers may not know what you and I and [a reporter I quoted] no doubt do, to wit, that until 1975 cloture required a 2/3 vote and that that was changed then to the current 3/5 (except in a vote on Senate rules which still needs 2/3). Even more interesting, however, is that the 2/3 was 'of Senators present and voting' and the 3/5 is 'of Senators sworn'. That, of course, is why it used to be that a filibuster could be continued only while the filibusterers kept their forces on the floor in sufficient numbers to constitute a third of all present, while since 1975 a Senator at home in bed is a vote to continue the filibuster. That changed everything and there is no more strategy involved, i.e., you have 60 votes or you don't and persuading an opponent to abstain or absent himself for an hour is without effect.
Dennis Hutchinson, a longtime friend who teaches constitutional law at the University of Chicago law school, adds:
Of course the problem is that the Senate mis-used its powers under Art. I, §5, cl., 2 to "determine the rules of its proceedings" when Mike Mansfield allowed tracking such that two items of legislation could be on the floor at the same time and one could be filibustered without stopping the entire proceedings. And since a two-thirds votes is necessary, under Senate rules, to change Senate rules, this problem will be around for awhile.
You can read more about the consequences of this 1975 reform from Thomas Geoghegan (for the record, also a longtime friend). And no doubt you'll have further chances in this space! For now this last thought, from a reader with family ties to George Norris, the long-time "progressive Republican" U.S. Senator from Nebraska:
[My ancestor] Senator Norris filibustered the old fashioned way, as it were. (His stand that no politician should invest in any asset except US bonds to avoid any bias also contrasts sharply with politicians today.) I did want to point out that, if the Democrats lose the Senate, then I predict that the Republicans will simply change the rules*, thus eliminating the problem. At that point they will cheerfully switch sides, and then ram it down the throat of the Dems. My favorite line of Krugman's is that the Republicans "are serious men", by which he meant that they played a tougher, and longer, game than their opponents.
* To clarify, changing rules during a session requires a 2/3 vote, but it is generally understood that every two years, at the start of each new Congress, each House can set rules for itself by majority vote.