We cannot however be regarded even at this time, as one homogeneous mass, in which every thing that affects a part will affect in the same manner the whole. In framing a system which we wish to last for ages, we shd. not lose sight of the changes which ages will produce. An increase of population will of necessity increase the proportion of those who will labour under all the hardships of life, & secretly sigh for a more equal distribution of its blessings. These may in time outnumber those who are placed above the feelings of indigence. According to the equal laws of suffrage, the power will slide into the hands of the former. No agrarian attempts have yet been made in in this Country, but symtoms, of a leveling spirit, as we have understood, have sufficiently appeared in a certain quarters to give notice of the future danger. How is this danger to be guarded agst. on republican principles? How is the danger in all cases of interested coalitions to oppress the minority to be guarded agst.? Among other means by the establishment of a body in the Govt. sufficiently respectable for its wisdom & virtue, to aid on such emergences, the preponderance of justice by throwing its weight into that scale. Such being the objects of the second branch in the proposed Govt.
When Madison talks about the "leveling spirit", he's talking about socialism. Back in the days of the Founding Fathers, those who would use government to reach into your back pocket had not yet decided to call themselves 'socialists'. They called themselves "levellers". As in, levelling the playing field, levelling people's incomes, levelling the amount of materialistic wants in everybody's house. Socialism.
Madison also specifically asks how to protect against this danger since it is so much a threat to any system that they wanted to "last for the ages". That's very forward thinking. Our Founders did not want government reaching into Peter's pocket for the lone/express purpose of giving to Paul. Paul did not earn that, so he should not get it. As Benjamin Franklin pointed out, that sort of activity is tyrannical. Taking that which is not yours is tyrannical.
He was right to worry about this sort of tyranny. Where we are at now, people no longer "secretly sigh" for government to equally distribute people's earnings. They openly proclaim that government should take, and take plenty.
Progressives should not be allowed to get away with casting this as something new. Wealth redistribution - tyranny - is older than liberty. These are not the "new ideas" that they proclaim to be the heralds of.