How would Cassius and Brutus from Julius Caesar fare as leaders/politicians in the 21st century?
Brutus and Cassius would not succeed in our century for the same reasons they failed in their own: Their failures resulted from personal weaknesses, not particular circumstances.
Brutus failed as a political leader because he was a poor judge of character. He could not assess the political motives of others, even those of his closest friend Cassius. As a result, Brutus was easily manipulated in the real world while he acted upon his philosophies that were rooted in his vision of an ideal world. Regardless of the political times in which Brutus might find himself, he would not succeed. He was too honorable and idealistic to survive in the game of power politics.
Cassius would not succeed either. Even though he was politically astute and clever in his ability to manipulate others to get what he wanted, Cassius lacked the strength or the will to act upon what he knew to be true. On at least three occasions, Cassius bowed to Brutus' judgment, even while he knew Brutus was wrong, and each time this led to disaster. For instance, if Brutus and Cassius had dealt with Antony as the deadly enemy Cassius believed him to be, the outcome of Caesar's assassination might have been entirely more favorable to them. Also, once he achieved power, Cassius proved to be a grasping, corrupt leader, one who took bribes. In a modern political setting in which politicians' personal finances are closely scrutinized, this in itself might have led to his downfall.