Presidents are going to nominate Supreme Court Justices who, roughly, correspond to their political beliefs. There is no use complaining about that. And there is a solid case to be made that, where the nominee is not otherwise unqualified or (perhaps) a shrieking radical, members of the opposition should not actively block the confirmation.
That norm had been fraying for quite a few terms now. But the treatment of Merrick Garland has completely annihilated it.
The fact of the matter is that Democrats legitimately feel robbed of a Supreme Court Justice right now. There was a vacancy in a Democratic administration. The nominee was a liberal, but not one that even Republicans contended was either unqualified or some sort of radical. Garland was a Breyer, not a Kennedy, but he wasn't a Reinhardt either. There wasn't even the pretense that GOP obstruction had anything to do with Garland's own merits as a potential Justice. They just decided that they weren't going to vote for, or allow a vote on, or even consider someone a Democrat nominated. Hell, Ted Cruz went as far as to suggest Republicans could justly prevent a Democrat from ever appointing anyone to the Supreme Court ever again.
Given that, how can anyone look at Senate Democrats and tell them with a straight face that they shouldn't just line up to block whomever Trump nominates? It's not just "they did it first," it would also function as a form of restorative justice. Again, from their vantage point this is a stolen seat -- full stop. And we just learned there is no political consequence for this sort of obstruction.