I believe that Geary knew he was Mike's puppet,and would never risk angering the Corleone's. He knew first hand what they were capable of, and in jail or not,Mike could drop the hammer on him.

Being a Senator, Geary was probably immune from being killed,given the incredible heat that it would bring,but his weak spots were women and greed.

So maybe next time,instead of one dead hooker,he gets caught with one (or two) live ones. The newspaper people and cops on the Corleone payroll make sure it's not covered up,and Bada Beep Bada Boop, no more Senate job. Or, he gets set up taking bribes,maybe a little laundering scheme to avoid taxes, and same result.

So as long as Mike needed him, Geary was his man.