The blocking of Judge Merrick Garland's nomination to the Supreme Court was a calculated risk on the part of Republicans. He's an eminently centrist pick who raised no real objections among Republicans other than "He's not a radical right wing extremist." But the gamble seems to have paid off with the victory of Donald and now it's up to the Democrats to flip the script and block anyone that is not Garland.
The legitimacy of the Supreme Court relies on it being free from political tampering. That is why appointments are for life and it's almost impossible to remove a sitting Justice through impeachment. But in their reckless pursuit of power at all costs, Republicans are on the verge of tainting the Supreme Court for a generation (if not permanently). By unconstitutionally keeping a seat vacant, they are literally stealing control of the Court from the electorate and Democrats should respond accordingly.
The "justification" for blocking Obama's appointment is that the people should have a say by voting for the next president. Aside from the fact that they already did when they elected Obama twice, the voters still made their will known by giving Hillary Clinton a solid win in the popular vote (2.4 million and still rising). Republicans can bitch all they want but they themselves demanded the people have a voice, not the electors of the Electoral College.
And after Donald is inevitably impeached by Republicans or he simply quits because it's too much of a hassle to be president (or falls over dead because he's an obese 70-year-old who doesn't exercise), Mike Pence will not have a legitimate claim to appoint any Justices, either. The people did not elect him to the presidency, after all, and that's the most important thing according to Republicans.
Right now, the Supreme Court is the only thing standing between us and Republicans permanently rigging the elections so they never lose again. They've already used the weakened Voting Rights Act to give themselves a win they did not earn. Trump's margin of victory in several states was narrow enough that blocking mostly Democratic voters clearly had the desired impact.
By tilting the Court to the right again for a generation, there will be no legal recourse to undo the coming damage to our electoral system. That alone should be enough for the Democrats to find their spine and wage scorched earth war on every single nominee the GOP offers up.
Republicans threatened to block Clinton's nominees for the next four years if she won and it's vital that Democrats actually go through with that threat. If that forces Republicans to use the "nuclear option" and do away with the filibuster, so be it. They can try to justify their legislative tyranny to a country already sick of them. Republicans have become an existential threat to our democracy and it's time to stop pulling our punches.