Final answer:
The Federalist who warned about the dangers of political parties was George Washington. He did so in his farewell address, emphasizing the risks of divisiveness and lack of unity. The formation of political parties, despite Washington's cautions, took root early in American politics.
Step-by-step explanation:
The supporter of federalism who warned people about the dangers of political parties was George Washington. In his farewell address, Washington cautioned against the divisive nature of political parties and how factionalism could lead to a lack of national unity. Among the federalists like Alexander Hamilton and James Madison, George Washington stood unique in expressing these concerns plainly. On the other side, figures like Thomas Jefferson represented the emerging Republican Party, which opposed many Federalist policies. The development of political parties, despite the initial resistance of many early founders, effectively became a hallmark of the American political landscape during this period. This partisanship is evident during the 1796 election, where the Federalist John Adams became president with the Republican Thomas Jefferson as his vice president.
It's also important to note that Patrick Henry was not a Federalist; he was a strong opponent of the Constitution's ratification without a Bill of Rights.