I always took Geary's speech in the hearings as total fence-sitting. He said just enough to sound like he was tough-on-crime Senator who wanted to expose Michael, but at the same time not enough to bring down the Corleones. He sounds tough questioning Willie C., but asks questions that keep Michael insulated (the buffers.) Sure he knew Frankie P. was going to be the surprise witness, but he never gets to question him, so we don't know what kind of dodge he would attempt. His speech before leaving for his own committee can imply that Michael Corleone is NOT one of those rotten apples.

I think at that point Geary is not sure of the outcome, so he craftily plays up to both sides so no matter who comes out on top he can claim support for the winner. Spineless slimbag.