I blogged again over at Plum Line today about cloture votes and filibusters (they're not the same thing!) and received a request in comments to explain. So here's a primer that goes into a bit more detail than the last couple of times I've written about it.
A filibuster generally has been an effort to defeat something by using available parliamentary maneuvers; most famously that's been talking endlessly, but any maneuver qualifies. Or, to put it another way, what we're really interested in here is any kind of obstruction by the minority -- any minority, partisan or otherwise. We can use "filibuster" and "obstruction" interchangeably; it's best to think of them that way, rather than to try to figure out which set of minority obstruction should count as "filibusters" and which should not.
Obstruction in the Senate can be stopped by cutting a deal; by waiting it out; or by overcoming it using cloture, which since the 1970s has required 3/5 of all sitting Senators, or 60 votes. It used to be that waiting it out was a viable strategy, but for a variety of very good reasons it really hasn't been for some time, at least since the 1960s. Why? Because as the Senate's workload increased, floor time because far too valuable for the majority.
So cloture votes would seem like a good measure of filibusters, right? Well, no. After all, legislative majorities rarely like to bring something to a vote if they know they'll likely lose, except rarely for the spin value. Therefore, we might expect lots of successful filibusters -- those that anywhere from 41 through 49 Senators support -- to rarely draw cloture votes at all. The majority will likely just drop the measure, or perhaps enter into negotiations to modify it.
But that's not all! Single-Senator or small group filibusters might not draw a cloture vote, either. Sometimes, the majority won't want to spend even the time that a "successfully" overcome filibuster will take, and so they'll drop the measure before it hits the floor. Other times, they'll cut a deal, preferring to compromise rather than spending valuable floor time forcing something through.
Given Senate rules, which allow for multiple opportunities within consideration of a single bill for obstruction that can be stopped by cloture, it's also possible for cloture votes to overstate the number of filibusters. It's also possible that the majority can schedule and then actually go through with a cloture vote even though no obstruction is intended. After all, normal debate might last for a while without any intent at all of obstruction, but the majority might believe that obstruction is intended (or just fear it) and move to cloture anyway.
To put it all together: cloture is only one means of ending a filibuster. Counting cloture votes counts only one of the ways of fighting a filibuster, ignoring the other ways -- and ignoring filibusters that win without being contested.
Again: when minority obstruction was relatively rare, cloture votes weren't the worst way to track changes over time, and had the advantage of being relatively easy to come by. No one thought, or should have thought at least, that they measured exactly how many filibusters there actually were, but at least one could argue that they gave a fairly useful apples-to-apples comparison across time. But given the true 60 vote Senate, we know that by definition the minority is insisting on supermajorities for everything, which means everything is being obstructed, and so there's no need to use anything other than 100%.