The Dirty Deal was that McConnell, Romney, Thune and their ilk agreed to throw the House and presidential elections so that they would control the opposition to Biden and thus Republican Party.
Mitch threw the House to the Dems in 2018 so Trump would be hamstrung for the remainder of his term. How did Mitch throw the House to the Dems? By giving an interview about three weeks before the election saying that Social Security needed to be cut. He said this with just enough time for Dems to use his words in ads all around the country. He had to know they would do that.