Final answer:
James Madison's main idea is that a larger republic will have a variety of interests which makes it difficult for any single group to unify and oppress others, consequently protecting the rights of individuals and minority groups.
Step-by-step explanation:
The main idea in the passage from The Federalist Papers by James Madison is that a larger republic with a more extensive variety of parties and interests is less likely to succumb to the tyranny of a single majority faction. Madison argues that an extended sphere for the republic, meaning a larger nation-state with more diverse interests, would naturally lead to a system where oppressive plans by a single group are harder to execute because coordination among a vast number of different factions becomes more challenging. Additionally, in a large republic, a majority of citizens with a common motive to oppress others will find it more difficult to unite and enact such plans, thereby better protecting the rights of the individual and minority groups.