In every government there is some power stronger than all the rest; and the power which is strongest tends perpetually to become the sole power.
 John Stuart Mill, Considerations on Representative Government (1861). copy citation

Context

“This portion of the assembly would also be the appropriate organ of a great social function, for which there is no provision in any existing democracy, but which in no government can remain permanently unfulfilled without condemning that government to infallible degeneracy and decay. This may be called the function of Antagonism. In every government there is some power stronger than all the rest; and the power which is strongest tends perpetually to become the sole power. Partly by intention and partly unconsciously, it is ever striving to make all other things bend to itself, and is not content while there is any thing which makes permanent head against it, any influence not in agreement with its spirit.” source