to clash or to confront.

Oh. Ok. That’s very particular. Thank you! @Bill(I’m new in this app, and i don’t know how to @ you so I have to type it😅)

"To knock heads" is an informal figurative expression that means to impose some type of consequence in response to bad behavior; the consequence could be verbal only, such as a reprimand, or it could also include some type of punishment or damage to their reputation. Usually the people who "knock heads" have some type of authority over the people whose heads they are knocking: a parent knocks their children's heads, or a teacher knocks their student's heads, or a boss knocks the heads of the people that they supervise. In this context of this sentence, it means that ordinary citizens will have to use similar methods to force politicians to understand how ordinary citizens are angry with the politicians failure to cooperate better with one another.