In the first scene of Act 3, Macbeth speaks with two men (named First Murderer and Second Murderer) in order to accomplish this murderous end and stay in power. We discover that he spoke with both men earlier, and at that time he lied to them by blaming Banquo for the source of all their misfortunes. In this scene, he tells them that Banquo has deceived them, and robbed them of what is truly theirs. He asks them if they are men, specifically the kind of men who would not be afraid to take justice into their own hands. When they affirm that they are, he tells them precisely where and when they can find Banquo and his son.
He never actually tells them to kill Banquo. Instead, Macbeth just gives them false reasons to hate Banquo, hints at what should be done, riles them up and, tells them precisely where Banquo is going to be. They then leave and murder Banquo in a later scene.
… I'm guessing that the majority of Republican Senators would vote to acquit Macbeth in regards to being at fault for Banquo's murder.