James Madison was a federalist leader. The need for a Bill of Rights did not convince him. He thought that governments were capable of securing freedoms without the need for a federal mandate imposed. He was worried about the problems that a document, that at first seemed unnecessary, could cause, and so he tried to appeal to the anti-federalists to give up this effort to write it.
But the debate grew tension between federalists and anti-federalists as well. Madison thought that suppressing a Bill of Rights from the Constitution could lead the anti-federalists to abandon the drafting effort altogether, and when he saw that his goal of building a form of self-government was in danger, he put aside the disagreements he had with the anti-federalists.
Madison thus becomes an advocate of the Bill of Rights, arguing that he would not only educate people about their rights, but would also be a vehicle to protect them from future oppressive governments.
Madison harshly criticized the efforts to add a Bill of Rights to the Constitution at first, regarding it as unnecessary and going even farther referring to it as a "nauseous project of amendments". Later on, however, he recognized that the lack of a Bill of Rights would give Anti-Federalists an excuse to oppose to the Constitution, claiming it wouldn't properly guarantee individuals liberties such as freedom of speech and religion without it. Madison argued that these liberties weren't compromised by any means, as the government wasn't allowed to take any actions to threaten it that it wasn't explicitly allowed to. Also, a majority of states already had a bill of rights of their own and a Federal one would just be redundant and unnecessary.
Madison ended up changing his mind for political reasons, when his attempts to silence the opposition weren't enough. Even though he still thought the Bill of Rights was unnecessary, he pushed forward to add it with the aim of preserving the stability of the Nation.
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