Our Constitution establishes a set of checks and balances between the three branches of government. This prevents any one branch from dominating and rendering one or both of the others irrelevant. The relative balance tends to shift over time, typically between the executive and the legislative branches, but overall one is ineffective without at least some support from the others.
While factionalism was already present during the drafting of the Constitution (most notably the Federalists and the Anti-Federalists), these factions were not included in the design of government. This stands in contrast to parliamentary systems, where political parties are made an explicit part of the government.
Here, the parties form what might be called extra-Constitutional branches of government. The checks and balances on the parties that are absent from the Constitution are instead embodied in the procedural rules of the House of Representatives and the Senate. In general, the impact of these rules is not felt so long as different branches of the government are controlled by different parties. The tensions between parties and the tensions between branches tend to combine, restraining imbalance through partisanship.
At this point, it is appropriate to take a closer look at partisan politics. Taking a charitable view, we will assume that all of our elected officials have the best interests of the citizenry at heart, and are not motivated by personal ambition or power. Entry into politics is typically through one of the major parties, so by the time a politician reaches national office, he or she has had many years working within the party structure. While individuals often have personal views of what is good or bad for the country that would be difficult to fit into the ideals of one particular party, they often identify more closely with one party than another.
Identifying with a party is fine, of course. The tendency of a partisan, however, is to come to view the positions of his or her party as the best positions for the country as a whole. Working for the common good thus becomes working to ensure that the party acquires or maintains control of the government, since this puts it in the best position to implement the policies that are "right."
In politics, there is rarely a unique definition of "right," however. If one party is "right," the other must be "wrong," albeit misguided rather than malicious. When the opportunity presents itself, it is therefore advantageous for a party to exert its influence as completely as possible. The opposition will, of course, eventually see the error of its ways and concede that the policies implemented truly were the ones the country needed.
In this way, parties tend to arrogate to their positions an almost divine right. This same arrogance afflicts the different branches of government as well, but there the Constitution mitigates its effects. When one party controls all or substantially all of the government, its arrogance is checked only by its respect for the established rules of engagement with the other party.
Of course, we now find ourselves in the circumstance where one party controls the executive, the legislature, and a respectable fraction of the judiciary. With the presence of judicial vacancies, it is natural that this party wants to fill those vacancies with judges sympathetic to the party's ideals.
Federal judges are a unique group within the government. Unelected, they serve terms considerably longer than any elected officials. This makes their selection a matter of great significance, as they wield considerable influence.
Approving a judicial nomination requires only a simple majority in the Senate. Clearly, if put to a vote an nominee favorable to the controlling party will easily win confirmation. Committees tend to be stacked in favor of the controlling party as well, so committee hearings give the opposition little leverage to object to nominees.
The opposition is left with one option with which to object to judicial nominees: prevent the nominee from coming up for a vote. This is more than a courtesy to the opposition; it is the only voice the opposition has in the process. While much of the Senate's business can be conducted with only a simple majority, important decisions such as filling the Federal Bench deserve a check by the minority.
A common argument for preserving the minority's right to filibuster judicial nominees is that eventually the tables will turn, and the party currently in the majority will find itself in the minority and faced with a similarly distasteful situtation. While this is a perfectly reasonable argument, I would like to present another. The purpose of checks and balances is to rein in political arrogance, and to acknowlege our inherent fallibility. The longer the results of our decisions last, the greater the humility we should exhibit when making them. For this reason, if for no other, we must preserve the partisan checks and balances established not by Constitution, but by civility, by collegiality, and by decency.