Mueller, if he has any integrity or honesty, already realizes that the "crimes" allegedly committed by Trump are actually crimes that were committed against Trump. And that some of the anti-Trump conspirators, i.e. Strzok, were part of his investigation team. Mueller must now make a decision: either he can go down with the anti-Trump criminals his investigation was created to protect, or he can confess that he has been used by these people for unethical purposes and become a witness against them. We'll soon find out which road Mueller wants to travel.
Same for Rosenstein, but is someone who has a long personal history deeply entwined with Mueller and Comey.
If this was all above board, and the players were honest, then Rosenstein would have announced that the Mueller special counsel was going to take the investigation of wrongdoing against Trump under the umbrella of the current investigation of the election. But he's not done that, probably because he knows Mueller's would screw it all up.
So now we get these Senators calling for a second special counsel, to vet the evidence the first special counsel apparently didn't, and still isn't capable of correctly investigating. What a fiasco.