Not just landlords, but everyday home owners too. People who got a $X00k mortgage on a house by-and-large are not going to actively support policies which may reduce the asset backing that loan.

There would absolutely be long-term benefits to lowering housing costs in parts of the country, but some people are going to have to feel the short-term pain to make that happen.

I think anti-profiteering laws could be helpful. If you bought land and built a house and want to live in a quiet neighborhood until you die, fine. But maybe if you vote against expanded housing you don't get to sell your house for more than you paid for it, or maybe don't get to sell it at all while you're alive.

Separating the homesteaders from the investors would do a lot of good.

I think this is a convenient caricature of why people want to keep single family zoning, but it doesn't match my experience. I am one of those people that favors single family zoning, and no one I know that shares the same view favors low-density zoning to drive value. Not one. The reason I hear consistently is that people want to preserve their way of life and quality of life. Introducing new zoning and higher density works against that. It brings an influx of newcomers that change local politics, it causes additional traffic beyond what driving infrastructure can support, it soaks up parking and makes businesses/neighborhoods less accessible to those living outside the immediate vicinity, it makes parks/amenities more crowded, it changes the sense of community, it affects safety, and so on. Those things are important to many people, not just current home owners, and I think it particularly matters to those who have lived in an area for a long time and helped make it what it is.