Anti-Federalists wanted to protect states' and individuals' rights. They opposed the adoption of the U.S. Constitution because they feared the strong central government created by the Constitution could become corrupt and tyrannical.
Anti-Federalists admitted that the Articles of Confederation needed to be changed, but they believed the Constitution, as it was written at the time, threatened to create political corruption and make the federal government too powerful. Their concerns and agitations helped force the addition of the Bill of Rights.
Perhaps the most powerful arguments raised by the anti-Federalists were those about the lack of protection of individual liberties present in the Constitution before the addition of the Bill of Rights. Most state constitutions of the time had followed the model set by Virginia and had explicitly detailed what individual rights could not be taken away by the government. Many people of the time thought this was a great improvement over the British constitution, which relied on unwritten protections. Anti-Federalists believed the delegates of the Philadelphia Convention had not included a bill of rights in the Constitution because they wanted to take away what ordinary people had gained in the Revolution.
A diverse group of people considered themselves anti-Federalist. Well-known proponents of the movement included Patrick Henry, George Mason, James Winthrop, Samuel Adams and Thomas Paine.