Any government which is ruled by the preferences of the majority is in a position to squash individuals who hold opinions contrary to that majority.
Mill is not referring to political power, but, rather, to the power of the majority of any given society, using means other than legislation but no less coercive, to prevent the individual from freely expressing themselves in their own chosen lifestyle.
He means both the majority acting via the official political sphere as well as the methods of imposing majority socially accepted conventions on individuals via less official means.
Any government based on majority rule will result in a set of inconsistent legislation, also known as the 'voters paradox'.
Mill's deductively valid argument for liberty of thought, speech and even lifestyle has which of the following as premises (there is more than one correct answer)?
Either the opposition's views are true, or they are false, or they have at least some truth.
If the opposition's opinion is true, then we all benefit from having that truth remove us from our false opinion.
If the opposition's opinion is false, then we all benefit from having our belief challenged and gaining the firm conviction that comes from understanding more fully our beliefs and successfully defending them in rational debate.
If the opposition's opinion is partly true and ours only partly true, then we all benefit.
Mill thinks the justification for his stance on liberty is at bottom based on:
The rights of the individual derived from the state of nature to be free to do what they see fit for their own survival.
Natural law, in Locke's sense of a moral law of God.
The greatest good for the greatest number of people.