Still, while reading Federalist 58, it's pretty easy to see what the Founders would've thought about the filibuster: They wouldn't have liked it. We can surmise as much when James Madison grapples with whether the Constitution should've required much more than a quorum for the House of Representatives to vote on weighty matters. Madison didn't like the burden that would create. Why? It would enable a minority of Congressmen to block legislation simply by not showing up:
It has been said that more than a majority ought to have been required for a quorum; and in particular cases, if not in all, more than a majority of a quorum for a decision. That some advantages might have resulted from such a precaution, cannot be denied. It might have been an additional shield to some particular interests, and another obstacle generally to hasty and partial measures. But these considerations are outweighed by the inconveniences in the opposite scale. In all cases where justice or the general good might require new laws to be passed, or active measures to be pursued, the fundamental principle of free government would be reversed. It would be no longer the majority that would rule: the power would be transferred to the minority. Were the defensive privilege limited to particular cases, an interested minority might take advantage of it to screen themselves from equitable sacrifices to the general weal, or, in particular emergencies, to extort unreasonable indulgences.Some of my conservative friends defend the filibuster as a minority legislative defense against the tyranny of the majority. But it's pretty clear the Founders—or at least Madison—weren't interested in giving the minority the power to block legislation, seeing it as a reversal of "the fundamental principle of free government."
Combine this with Federalist 22, in which Alexander Hamilton railed against the idea that two-thirds of Americans would "submit their interests to the management and disposal of one third"—roughly the ratio needed for the Senate to sustain a filibuster—and it's sure seems clear the practice is both undemocratic and a betrayal of the Founders' vision.
A conservative friend says I may modify my position on this when I read further into the Federalists. Right now, though, the filibuster isn't really faring well.