I've been looking at the history of FmHA recently. Post WWII it started off mostly lending to farmers, operating and ownership loans. Over time successive legislation gradually widened the scope to include lending for housing, for community facilities, to towns <2,500 people expanding to 50,000.
I suspect, without researching it, that most if not all of these expansions went through without too much partisan controversy or debate. I see the "iron triangle" at work: the FmHA bureaucrats, the lobbyists, and the Congressional committees working together to push the changes through and with support from rural representatives of both parties. I don't see ideology as playing much of a role, except a generic pro-rural development stance. Just guessing, I'd think partisan politics probably comes into play more when a new agency is being created, when it's not a matter of adding functions to an existing agency which already has a bureaucracy with ties to Congress and to interest groups but creating something mostly from scratch.