Everyone makes good points, and I must say I agree with TB that Geary saw the Senate hearings as a way he could get out from under Michael's thumb. Too bad this plotline did not get further developed because then Geary could have been included in that killing montage at the end of the movie, set up as follows:
Michael (to Neri)- And Geary?
Neri - He comes back to Vegas every other weekend to make a big show of playing family man with his wife and kids.
Michael - Then meet him in Washington. I don't want his family involved.
Tom - You want to gun down a U.S. Senator?
Michael - Tom, he never wanted to go along with us, you should know that. Rocco, before you go to Miami, make a stop in D.C.
Rocco - No that's an easier one.

Geary was forced into submission. He was a bigoted jerk, who was a crook and a hippocrite. And not a family man, which Michael would have hated. Contrast that to all the politicians who got invited to Connie's wedding, made lame excuses about how they couldn't make it, but all sent gifts to her just as well. Now thats how bribery works.