Court-packing is perfectly legal. The Constitution (foolishly, IMO) allows Congress to set the number of Supreme Court justices.
But it has never happened. One reason being that the Democrats in the old days differed from Republicans only by a little. Think Truman and JFK.
But today’s Democrats are not your grandfather’s Democrats. Think Bernie and Liz Warren. So you can bet they’ll pack the court if given the chance.
I think the only thing that saved us from “card check” unionization and “cap and tax” energy policy in 2009-2010 was that the Dems got bogged down with Obamacare and the clock ran out.
Now that would be a constitutional crisis and get the sheeple's attention I would imagine.