When the news came out Saturday that Supreme Court Justice Antonin Scalia had died, conservatives mourned his passing and recalled his legacy while some progressives actively celebrated his death. However, the substantive discussion which quickly developed surrounds when and by whom Scalia's replacement will be appointed.
In a sense, there's no question on this. The constitution lays out that the president nominates justices for the Supreme Court, and the way that the Senate has always carried out its "advise and consent" function in the process is by voting to confirm or reject presidential appointments. However, we're already into the presidential election cycle, with the possibility that President Obama's successor will be a Republican, so naturally the Republican majority in the Senate is already suggesting that they will simply refuse to confirm any nominee Obama puts forward, waiting to see the results of the election instead. His has provoked howls from Democrats, who accuse Republicans of ignoring the constitutional duties of the Senate for partisan reasons.
The issue which this whole controversy underscores, and yet which is mostly being talked about only tangentially, is the unique place that the Supreme Court has come to fill in our government over the last 30 years. (Arguably with Roe v Wade as one of the key inciting incidents.)
Control of the court allows both sweeping non-legislative change, and also allows for the blocking of major pieces of legislation or executive regulation which seek to change the way that the country has historically worked.
For the last 25 years or so (arguably since Planned Parenthood vs Casey) the court has maintained a tenuous balance with at any given time four reliable "conservative" votes and four reliable "liberal" ones, and Kennedy providing the swing in between.
Having either party get to replace one of these seats with someone of the opposite persuasion is a potentially massive shift in how the country works for the next 20+ years.
If RBG had chosen to retire this year, we would not be having the same fuss about Obama replacing her. Sure, conservatives would love to get the chance to replace a reliable progressive vote with a reliable conservative one (or to frame it more clearly in the way that these terms apply to the court: a justice who rules on what she thinks the constitution ought to say with one who rules according to what he believes the constitution does say) but I don't think that if one of the liberal seats were coming open at this moment we'd see the same readiness to fight. It is the fact that one of the conservative leading lights of the court which is up for replacement which is causing the massive concern, and it's hardly surprising that this is so. This is the same reason that Sen. Schumer back in 2007 announced (when there wasn't even a vacancy) that if Stevens or RBG stepped down, the Senate Democrats would prevent any nomination from going through until after Bush's term.
Both parties want to flip the court to be decisively in their favor, but short of that they feel even more strongly about not losing the advantage they have. But in a sense it's surprising that the stalemate has lasted as long as it has. The timing of party switches in the white house, the defection of several Reagan appointees to the liberal side of the court, and the long tenures of many recent justices have all helped to maintain an equilibrium for a long time and people have in some sense come to rely upon it. Sure, it has to break some time, but if the break came with a recently elected president and a congress controlled by the same part, it would at least have a certain fairness to the breaking. There is no reason for Republicans who control congress to collaborate with President Obama in creating what could well be a 20+ year liberal lock on control of the court when it at least remains possible that a Republican president will be in the White House in less than a year.
However, unless our country moves away from relying so heavily on judicial fiat to break stalemates which our legislature is unwilling or unable to, control of the Supreme Court will remain in many ways control of the government, and no one wants to give that up. And the angst is all the higher because it's an aspect of the government which is very hard to influence through the democratic process.
Right now some Democrats are offering pained editorials about how unfortunate it is that Republicans won't step away from politics and confirm whatever qualified nominee Obama puts forward. If the function of the court would de-politicize, if it would step decisively away from the kind of highly political rulings it has indulged in ranging from Roe v. Wade to Obergefell v. Hodges, we would not have to experience these kind of highly partisan wranglings over filling Supreme Court appointments. However, it's impossible to imagine the court moving away from that kind of function, both because the door has been opened (and is now hard to close) and because the rest of our government is at times complicit and punting hard decisions to the court. So for now, we're stuck with every Supreme Court nomination being fraught, and a case like which would potentially involve a massive philosophical shift for the court to be a situation for all out partisan war.