Mike, I figure Trump has been persuaded by his staff and lawyers that firing Comey places him in jeopardy of impeachment for obstruction of justice. He grasps that firing Sessions would compound that jeopardy, but he has managed to delude himself into believing that if Sessions can be pressured to resign, Trump avoids the image of obstruction. 

But this is pure delusion, since getting rid of Sessions is only a step to his objective to get rid of Mueller. The delusion continues, I figure, with the notion that if Trump's replacement AG were to fire Mueller rather than Trump, he would avoid the image of obstruction in attaining his goal. 

Is it coincident that Trump utter determination to get rid of Mueller is associated with his "red line" (Mueller investigating Trump finances). I am more and more inclined to the view that Trump's ultimate fear re Russia is the exposition of some sort of Putin financial stranglehold on Trump.

(Sure hope Mueller has his data backed up secretly offsite.