Why would Madison's support be needed to pass the financial plan?

Madison was a key proposer of the Residence Act, in where the Act asked for the US capitol to be moved to the Potomac River, which was indicative of the Southern states desire to move the US capital closer to the South. By supporting the Residence Act, Hamilton would be getting Madison on his side for the Assumption Act, and Hamilton could expect Madison to whip up southern congressmen to support the Assumption act, whom were desperately needed to get the numbers in order to pass in Congress.