I understand there are many people who major in math or physics and get into elite economics graduate schools. I'm sure that I will be able to find math professor who will with letters of recommendation that say I can successfully do research in math, but it would be hard for me to come up with such letter from economics professors. What do non-econonomics majors generally do for letters of recommendation?
Most committees are happy to see letters from mathematicians, although they might like at least one from an economist (and you probably should have at least one upper division course in economics if possible).
Just be careful. One eminent mathematician once wrote "X is not smart enough to be a mathematician, so I suggested he study economics." I don't think we admitted him.