There is a very simple path to packing. This is how it is done...

Packing the Court: Congress passes a law expanding the Supreme Court from 9 to, say 13, and Biden signs it. Then Biden appoints 4 justices, and the Senate confirms all of them. Voila, the Court is packed (as that term is used). The Democrats not only control SCOTUS, but they basically convert it into a legislative body (depending on who they appoint). That is a very simple path to expanding the Supreme court.

Packing the Senate: Congress passes a law admitting D.C. and Puerto Rico to the Union as States. 4 new Democrat Senators join Congress. Voila, the Senate is packed, and single party rule goes into effect, since it would be very difficult for the GOP to elect 4 new senators.

Of course, if they did ever regain control of Congress, they could do things like split Texas into 5 states, and split California up. Depending on how the lines are drawn, that would even things up by creating GOP Senators.

You can see how this game would eventually break up the Union.

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I am not at all nervous about the Democrats placing a justice on the Supreme Court like the Republicans did. That is normal.