These days the main questions in constitutional theory involve questions of interpretation – in particular, whether one should follow the original meaning of the Constitution or its evolving meaning or some mixture of the two. (One might also see this question as involving a related question – whether one’s theory of the Constitution should be about the document or about the practices of the country over time.)
But some years ago, constitutional theory focused on a different question – what is the dominant character of the Constitution. Some people argued that the Constitution is a fundamentally democratic document, essentially allowing a majority of the country to rule, except where the process of democratic rule is subject to infirmities. That is one way to understand John Hart Ely’s incredibly important book, Democracy and Distrust. The apparently majoritarian features of the Constitution – that legislation can be passed by a majority of the legislature – is the strongest support for this view.
Other people argue that the Constitution is fundamentally about protecting individual rights. These people can point to the Bill of Rights as well as the 14th Amendment as the main support for their view. These people might also point towards the Lockean roots of the political theory that inspired the Constitution.