Rosenstein testifies to the Senate Judiciary Committee
Feinstein, who was involved on the Senate Intel Committee when this was going on, is the ranking minority member. Lindsey Graham chairs the committee, a Republican whose bark is louder and more painful than his bite.
So far, my impression is that Rosenstein is not going to do well, and claim to be basically ignorant of anything. With regard to the FISA approval he signed, he claims he relied on what he was told was in the application -- meaning he is claiming he signed it without reading it. He can't identify anything McCabe lied to him about, even in hindsight, though he does admit there were some things that he felt McCabe was not candid about. So far he's trying to defend his decisions based on what he claims to have known at the time (versus what we know now). That's basically claiming ignorance of everything his subordinates were doing.
One sticky point is that he was asked if any of the investigative steps Mueller took relied upon the Steele dossier. Rosenstein says he doesn't know about individual steps. That's a dodge. Mueller got at least one renewal of the Page FISA which was almost entirely based on Steele's "information".