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LOL, this is a complete bluff. McConnell has spent years tinkering with Senate rules to ensure Republicans can pass any part of their agenda they want in ways that aren't subject to the legislative filibuster. That's why he's so scared of it going away. https://twitter.com/mateagold/status/1354130897805717507
To be clear: a Republican Senate without a legislative filibuster could do all sorts of horrible things. But nothing they can't do already through controlling other parts of government.
What McConnell is really afraid of is that a functional Democratic Senate will dismantle the backroom structures the GOP created to go around Congress for its agenda. That they'll overturn conservative court rulings, reform agencies they've taken over, undo voting restrictions.
And yes, a later Republican Senate can undo all these things. But it would force them to make unpopular votes. The whole point of creating these workarounds to Congress is so they don't have to take the blame when their policies hurt people.
