The question of whether the filibuster is Constitutional is in the news again, with a piece by David RePass in the Atlantic arguing it is not. I'll also point interested readers to an upcoming law review piece by Josh Hafetz called "The Unconstitutionality of the Filibuster."
I'm going to just link back to a longer post I did a while ago, and give you here the short version of why I think the filibuster is perfectly constitutional. Basically, it comes down to the "determine the rules" clause, which as I see it makes it pretty straightforward that Congress can arrange its business the way it wants, full stop (except, of course, when there's a explicit insistence on something, such as the supermajority vote in certain cases, or having a journal). Moreover, in practical terms, I think that's exactly the correct way to do things. Do those who believe the filibuster is unconstitutional really want the courts to step in and intervene? If not, what does it mean to say that something is unconstitutional but beyond the reach of the courts?
Hafetz, to his credit, does take on the "determine the rules" clause in his piece. He argues, however, that despite that clause a "determined and focused legislative majority must be able to get its way in a reasonable amount of time." His argument seems pretty weak to me, drawing on a hypothetical rule that would give incumbents a built-in numerical advantage for re-election, just as (he argues) the status quo has a built-in advantage of only needing 41%. Would it be unconstitutional for the Senate to decide that everyone is re-elected unless their opponent receives 60% of the vote? I'm not sure -- I don't know that the Senate's election judging power reaches that far. But if it does, it doesn't strike me as self-evident that such a foolish procedure would be unconstitutional. In fact, I believe that there are judicial retention elections that require a supermajority to throw the bums out. I certainly wouldn't support such a thing for Members of Congress, but that doesn't mean that the Constitution forbids it.
Beyond that, it's important to realize that no legislature runs on pure majority rule. Filibuster opponents object to supermajority requirements for passage, but well-placed minorities -- committee chairs, subcommitee chairs, majority party leaders -- have plenty of power to block measures that a majority of the chamber supports. A legislature that simply allowed any Member the right to bring any measure to the floor for an up-and-down vote would be courting chaos. It certainly wouldn't produce what Democratic supporters of reform want right now, which is a chamber more amenable to majority party control.
By the way, one final note on terminology. RePass says that a "silent filibuster" is an oxymoron, but that's incorrect. Hafetz is good on this: to filibuster is to delay in order to kill a measure, regardless of how it's done. Thus RePass is wrong to say that "throughout the course of the 111th Congress, if a bill did not meet the approval of the minority bloc in the Senate, it was threatened with a filibuster." What actually happened was that the minority party insisted on 60 for everything. Therefore, absolutely every bill and nomination was met with a filibuster. That was certainly new and important, and very different from the pre-1993 Senate. But not, I don't think, unconstitutional.