James madison was referring to when he wrote in federalist no. 62 that its necessity was "indicated by the propensity of all single and numerous assemblies to yield to the impulse of sudden and violent passions, and to be seduced by factious leaders into intemperate and pernicious resolutions."

The correct answer is the Senate.

He believed that faction leaders could gain the support of the senate and thus the factions would rule the congress which wouldn't be democratic and wouldn't work in the best interest of the people but rather of the leaders of the faction. This is just another reason why he explains that factions are bad and how they can ruin a country.