Justice Amy Coney Barrett is not the primary reason for their agitation.
"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 in placing three “conservative” appointees on the Supreme Court in one presidential term 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; goodness knows the Right has felt the same way under different circumstances. 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" . . .