James Madison's thesis in Federalist Paper Number 10 is that a strong national government is better able to guard against the destructive effects of special interest groups and factions than smaller republics. Madison wrote the essay to persuade the states to ratify the U.S. Constitution.
Madison defined faction as "a number of citizens, whether amounting to a minority or majority of the whole, who are united and actuated by some common impulse of passion, or of interest, adverse to the rights of other citizens, or to the permanent and aggregate interests of the community." He believed that factions are natural to human nature and that the best any government can do is to ameliorate their ill effects. Madison argued that this is impossible in a pure democracy, but a representative republic, where voters delegate law making to elected representatives, is better equipped to do this. In small republics, he went on, a smaller pool of voters means a greater probability that factions could form a majority to trample on the rights of the minority. In contrast, a larger republic means a larger pool of voters and candidates, and a smaller probability that any one faction could take power and oppress the minority.