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Re: Math Major Better Than Econ for PhD Admission?
So I am majoring in both math and economics. A couple of notes, at least from what I think:
1. A math major in itself is not really important; doing well (and learning the material well) in math courses is. If by doing the latter, you happen to get a math degree, great. But realize you can take all of the suggested math coursework without needing to get a major, and the law of diminishing returns does come to play. If you've taken the calculus sequence, linear algebra, probably theory & statistics, differential equations, pointset topology and two semesters of analysis, a semester or two of number theory and abstract algebra potentially required for a math major is not going to give you a whole lot of benefit beyond the courses you have taken. But if you love math and proof courses (which I do), go ahead! Doing well will help you, but doing poorly will hurt you.
2. If you decide to take these classes (which it seems like you are), your grades in Grad Micro really matter and your grades in Real Analysis really matter. These are probably your most important classes. Especially at at T30 school, these classes are challenging  you are taking them with PhD students  the 1520 students that a committee selected out of hundreds of applicants! If taking a math major causes you to rush your math classes so you overload yourself to complete requirements and prevent you from really concentrating to learn material and get an A in these classes, don't take a math major. If this is not the case, go ahead with a math major. Just be wise with your time.
3. Beyond sufficient math background (Calculus, Linear Algebra, Probability & Statistics, Real Analysis, and maybe Diff Eq and pointset topology  though some of this is not necessary, this is likely the upper bound of sufficient), more time on research / RA work for a professor / a thesis will probably do you more good than an additional math class.
So yes  sufficient math background + good grades in grad courses (in addition to research & letters & GRE) are great preparation. A math major is not necessary.