Jack Butler writes for National Review Online about the source of the political left’s anger toward the U.S. Supreme Court.

When Joe Biden finally answered, after several weeks of hemming and hawing, whether he would indulge the recently revived left-wing fantasy of adding justices to the Supreme Court, it was revealing. And not simply in the way that his response — “to put together a national . . . bipartisan commission” some months after Election Day — betrayed his decades as a political creature. (Have a problem? Form a “commission.”) For although this response has come to be seen as a kind of demurral, Biden still maintained that the Supreme Court was “out of whack.”

This is, in essence, the same belief held by others on the left, amplified during the confirmation of Amy Coney Barrett to the Supreme Court. Here as in other things, Biden seems to differ from the left primarily tactically, sharing many of its ends but aware that some of them — and some of its preferred means, such as Court-packing — are unpopular. But there remains the shared contention that what President Trump and Senate majority leader Mitch McConnell have accomplished … remains somehow illegitimate.

Superficially, this almost certainly proceeds from a sense on the left that they got played. Ignoring arguments of precedent, and the fact that, while McConnell undoubtedly played hardball, he did so well within his constitutional powers, many liberals are simply aggrieved at conservative political victories. And it is their right to be so aggrieved. … But that many on the left prefer to respond by seeking to dramatically alter the century-and-a-half-old configuration of the Supreme Court — something not even FDR could do as president — suggests something deeper is at play.

The fundamental source of this agitation is that the Left was convinced the Court would always be on its side, becoming its personal tool for achieving desired outcomes outside the electoral process.