True leader of the conspiracy

After the meeting of the conspirators in Act 2, Scene 1, the role of leader in the conspiracy seems to shift from Cassius to Brutus. I've always found this as an interesting part of the drama. Where is the turning point of this change. At what point does Brutus undoubtedly take over as the new leader? I believe it is when Cassius, Casca, Cinna and Metellus all believe that Cicero should be included. Brutus then speaks, offering his opinion to keep him out. After he says this, Casca and Cassius both seem to agree with Brutus, displaying his power among the men. How and why did this power-shift occur?

I think it a ploy to MAKE Brutus take the lead as he is the most respected Roman. Cassius is clever enough to hand over control to him