There is a lot of hypocrisy to go around.  The Democrats have always tried to thwart the appointment of the Republicans and visa-versa, especially in the era after Robert Bork, where the Democrats used ideology as a reason to oppose, and rejected the idea of Natural Law, to defeat this otherwise highly qualified nominee–I wold say the most qualified nominee of his time.

As Ann Althouse noted in her blog:

Obama is saying that he has the power under the Constitution to nominate someone to fill the vacancy on the Supreme Court even in the last year of his presidency and that there’s nothing in the Constitution that says he can’t. Of course, the Constitution also provides that the appointment isn’t complete until the Senate votes to confirm, and there’s nothing that says that power must be exercised within a certain period of time. These are opposing powers and the present struggle is political, which the President surely knows. Part of the politics is making claims based on interpretations of the Constitution, and each branch of government tends to interpret its own power broadly.

Yet more specifically, as Powerline notes correctly that the Senate does not have to hold a vote, at all, if senators don’t want to:

The Framers expressly based the Constitution’s “advice and consent” model on the approach used in Massachusetts, under the State’s Constitution of 1780. And, looking through years of archived nomination files, I found myriad examples of nominations made by the governor that received no up-or-down vote from the “Privy Council,” the body that provided constitutional advice and consent.

But the best evidence of the Senate’s power not to vote on nominations is found in the Framers’ rejection of an alternative approach to appointments. As an alternative to the “advice and consent” model, James Madison proposed a discretionary Senate veto. Under that plan, a president’s nominees would automatically be appointed unless the Senate mustered a majority vote against that nomination within a fixed number of days.

In short, Madison would have put the burden on the Senate, to affirmatively act to block a nomination. But the Framers rejected his proposal, and chose instead the “advice and consent” model, placing the burden on the president (and his supporters) to convince the Senate to confirm his nominee.

As the Adam J. White of the Weekly Standard writes:

And history reflects the Framers’ choice. Presidents have made 160 nominations for the Supreme Court. The Senate confirmed only 124 of them. And of the 36 failed nominations, the vast majority of them (25) received no up-or-down vote.

To that end, the Senate can structure its own rules to govern the advice-and-consent process. It had constitutional power to establish the filibuster system. It has constitutional power to abolish or reform the filibuster. And it probably should. But the Constitution leaves this choice to the Senate alone—just as it leaves the Senate free to decide whether to consider a president’s judicial nomination.

No matter the political machinations–and there’s lots of that to be outraged about, and that may interest us–the most important consideration here is the Constitution and the functioning of the republic under the rule of law.

No matter what Obama or Schumer, or Reid say, the Constitution governs what institutions may or may not do. To speak in manner contrary to it is irresponsible and places political expediency above the rule of law.  This means that one day, the Republicans may try to hammer through a nominee while Congress is in the hands of the Democrats.  It is still the will of Congress, which also represents the will of the people, to either confirm or not, or hold a vote, or not.

As Federalist #51 notes, the branches were made up to be counter each other in the service of freedom:

It is equally evident, that the members of each department should be as little dependent as possible on those of the others, for the emoluments annexed to their offices. Were the executive magistrate, or the judges, not independent of the legislature in this particular, their independence in every other would be merely nominal. But the great security against a gradual concentration of the several powers in the same department, consists in giving to those who administer each department the necessary constitutional means and personal motives to resist encroachments of the others. The provision for defense must in this, as in all other cases, be made commensurate to the danger of attack. Ambition must be made to counteract ambition. The interest of the man must be connected with the constitutional rights of the place. It may be a reflection on human nature, that such devices should be necessary to control the abuses of government. But what is government itself, but the greatest of all reflections on human nature? If men were angels, no government would be necessary. If angels were to govern men, neither external nor internal controls on government would be necessary. In framing a government which is to be administered by men over men, the great difficulty lies in this: you must first enable the government to control the governed; and in the next place oblige it to control itself.

The Congress in this instance is fulfilling its duty as stated by the Founders.  In some way it is checking a president who has run amok of his office.  The Senate is taking back some of what the Constitution allows and asserting its constitutional rights.