On the second to last day of the Philadelphia Convention, Saturday, September 15, 1787, in the second to last discussion of a substantive revision to the by then already printed up, nearly final draft , Roger Sherman of Connecticut raised his fear of the hypothetical possibility that, even if they were to faithfully follow Article Five's amendment method, three fourths of the states "might be brought to do things fatal to particular states." Large states, in short, might do away with small states. The ensuing debate encompassed diametrically divergent views, including in the course of exchange a drastic, absolutist proposal by Sherman to strike out Article Five altogether. Gouverneur Morris of Pennsylvania, however, proposed a compromise, unanimously agreed: "no State, without its consent, shall be deprived of its equal suffrage in the Senate." Becoming the last clause of Article Five, Morris' wording survives intact. It is the only proviso in the U.S. Constitution that cannot be amended.
According to Madison's notes this compromise "was agreed to without debate", which is more than a little strange given the delegates' clear understanding that the success of America's experiment in governance could not be known in advance. Things might not work out to perfection. Future practical considerations might indeed require different arrangements. Those delegates with a philosophical bent also could have shown at least some awareness of the a priori argument, a moral argument, that one generation trying to tie the hands of subsequent generations is a politically illegitimate act.
Perhaps the delegates were tired and wanted just to go home. Perhaps their deference to Roger Sherman overwhelmed their better judgment (Sherman, with Oliver Ellsworth, co-sponsored the Connecticut Compromise, and with James Wilson the Three-Fifths Compromise, without both of which no deal would have been possible between slave and free states). Perhaps in a last minute sorting through of loose ends the delegates overlooked the implications of giving states permanent equal suffrage in the Senate. We'll never know. To be fair, the delegates likely weren't thinking the problem through because they preferred to assume the permanence of their federal construction, the Senate being its linchpin. Why not throw everything else up for grabs, then, the Senate excepted? It's not so difficult to forgive the approximately three dozen older white men in attendance that day in Philadelphia. Far stranger, though, than their failure to debate an immodest scheme, let alone vote it down, is our long-standing indifference toward such a giant catch in the amending power.
The problem, obviously, is that the Senate is undemocratic because it represents geography, not people (38,000,000 people in California, for example, have the same number of senators, two, as 600,000 people in Wyoming). Moreover, as the population grows, the problem keeps getting worse. At the time of the Convention the population ratio of more to less populous states was about 11:1, now it's about 66:1, and so long as the population continues to grow (nobody thinks it won't), and continues to grow relatively faster in more populous states (a reasonable expectation), it could reach 80:1 by 2060 (a not unreasonable guess using U.S. Census projections), or 100:1 by the end of the century. At what point does a huge disparity become intolerable, if it isn't already? What remedies may be available?
On April 27, 1911, Victor Berger, a Socialist elected to Congress from Milwaukee, introduced H.J. Res. 79 proposing an amendment which would have vested all legislative powers in the House . As Berger put it in his preamble,
"[T]he Senate in particular has become an obstructive and useless body, a menace to the liberties of the people, and an obstacle to social growth; a body, many of the Members of which are representatives neither of a State nor of its people, but solely of certain predatory combinations, and a body which, by reason of the corruption often attending the election of its Members, has furnished the gravest public scandals in the history of the nation...."
He probably knew his amendment wouldn't make it out of committee, and it didn't. But as a symbolic gesture it helped push things along at a moment when popular resentment at the Senate's undemocratic nature had reached a fever pitch. Within seven weeks of Berger's proposal the Senate for the first time voted on an amendment providing for the direct election of Senators (that vote failed but a second vote in 1912 passed); by May 1913 the 17th Amendment had been ratified.
Berger should be remembered less for having dramatized the fight for the 17th Amendment than for his absolutely correct prognosis that it did not go far enough. This year being the hundredth anniversary of the 17th Amendment we have accumulated sufficient experience to say that direct elections have not brought an abundance of democracy to the Senate. The fact that there are questions whether direct elections have brought any democracy at all (a recent Brookings paper suggests that the Senate today is less responsive to voters than it was when state legislatures appointed their Senators) makes the case that early reformers misdiagnosed the problem.
Throughout the nineteenth century proponents of direct Senate elections slowly but steadily had gained traction. By the early twentieth century, mobilizing deep public passions for democracy, they had organized so effectively over this single issue that a sufficient number of states were poised to invoke the Article Five provision allowing states to call up a constitutional convention. It would have been the very first (and, to date, only) convention to consider amendments. From the point of view of the Senate this was a danger devoutly to be avoided — Trouble with a capital "T" — because nobody had any idea what such a convention might entail. There was no precedent, there were no rules under which it would operate, nothing, really, to control it. A "runaway" convention was a distinct possibility. Who knows what might have been amended?! The Senate capitulated. Reformers declared victory, disbanded, and turned their attention to other things; throughout the excitement of World War One, the Roaring Twenties, the Great Depression, and World War Two, the issue of an undemocratic Senate faded away.
If — a counterfactual — there had been a constitutional convention in 1912 or 1913, after taking care of the direct election business the convention might well have moved on to consider other amendments that limit the Senate's powers. Maybe the Senate shouldn't have any authority whatsoever over budget or revenue. Maybe advice and consent on executive appointments isn't strictly necessary. Maybe the Senate shouldn't have much power, period.
Those possibilities of whittling down the Senate's power through constitutional means, however, were permanently lost. Having once squandered the opportunity of a convention it is difficult to imagine mobilizing the required two thirds of state legislatures, again, today, for amendments aimed at a rather vague repurposing of Senate responsibilities. Nor would the Senate concur in a congressionally driven amendment to circumscribe its own powers. (If for some masochistic reason it did, an amendment still would have to pass three quarters of the state legislatures for ratification and since all the states except Nebraska have bicameral legislatures that means that of the ninety nine houses as few as twelve could block the process. In sum, those are astronomical odds against.)
Just as yesterday's reformers got sidetracked by the question of direct elections, today's headlines, like the filibuster, are red herrings. The source of the Senate's disease is, and has always been, its unrepresentative, anti-democratic structure as an institution. So we are stuck.
Given that our constitutional framework via Article Five's amendment method is not sufficiently elastic to allow for fundamental democratic reform the most principled avenue for change would seem to be an extra-constitutional convention, a "color" revolution, that by popular acclaim entirely replaces our system of government. This may not be much under discussion yet but it's not out of the question, either, not if your time horizon extends from several decades to a century. Before you break out laughing, think of Aleksandr Solzhenitsyn's experience with the Soviet Union!
There is one other way, a more immediate fix. Although it's a stretch to describe the U.S. government as a democracy (it's a plutocracy), and although the executive already has lamentably strong totalitarian tendencies, a counterintuitive further enhancement of executive power could perhaps steer us back onto a democratic track. Like skidding your car on ice, "embrace the slide." A President could become so frustrated with Senate dysfunction and obstruction that he or she shuts it down. This particular President won't, of course, because the Senate provides a convenient fig leaf for the things he wants. But a truly progressive candidate who campaigns on a suitably vague platform of Senate reform? Maybe. That person, though, would have to be an optimist! And once the Senate were disposed of congressional electoral districts might be normalized so that the party which wins a majority of the vote nationally wins a majority of seats. Some people would be made extremely uncomfortable, but so what? Over the longer term democracy could prevail.
The lesson here is that it is never wise to write unamendable provisos into one's constitution (or allow amendments that are unamendable). Such contraptions don't last forever even though it may take people a long, long time to figure out what to do about them.
 See James Madison's notes from the Convention, available at Yale's Avalon project. On Monday, September 17, one final revision was made (regarding the minimum size of congressional districts) before thirty nine of the delegates and the Convention's secretary signed the document.
 See the U.S. Senate website, historical minutes.