First of all, please note that this really should be about "money changers," not just "changers." That is important to understand because Coughlin is drawing a parallel to the story of Jesus ejecting the money changers from the temple in Jerusalem.
The money changers in Coughlin's speech are the big businesses and the elites. Coughlin believes that Roosevelt came into office promising to punish them and break their influence, only to give into them once in office. Coughlin is sounding a very populist note here, trying to portray Roosevelt (and all other politicians, really) as being in the pockets of the big business "money changers" instead of driving them out of the temple. This is why he mentions the huge debt and says that Roosevelt has given the bankers the unlimited power to spend and that Roosevelt will make the people pay it back through labor that will benefit the rich as well.
You can see this as similar to the Tea Party today with its anger against the bankers and the big businesses (and big government and other elites). Coughlin is appealing to that same sort of sentiment in this speech.