Pandora’s box is best left closed...
Yep. President Trump adds six and the next Democrat president adds nine. Sure, they might anyways but at least precedence hasn't been set.
This being said, holding on to the USSC is of paramount importance to liberals as they look towards an activist Supreme Court to aid them in their efforts to reform the Senate into a population-based institution.
That Pandora’s box will assuredly be opened wide should the Social Democrats take the Senate and the Presidency, ever.
If a communist wins in 2020, they are going to do it. If Trump were to goad them all into voting against doing that now, then they wouldn’t be able to justify it later.