thatrunnerkid, Duke is as flexible as Northwestern when it comes to changing majors. Most universities are. That’s the beauty of US universities. One isn’t locked in a major for the first two years of college.
fuzzzzzzster, Northwestern is only marginally better than Duke in Econ while Duke is marginally better than NU in Political Science. I doubt the difference will be felt by an undergraduate student. Between those two great universities, I would recommend going for fit. For example, Northwestern has an excellent 2 year program called the MMSS (Mathematical Methods in the Social Sciences), which can be taken in conjunction with the Economics or Political Science (or both) major. Northwestern also has two Certificate programs offer by Kellogg which are also great. I am sure Duke also has great programs to consider. Those are the details I would focus on…assuming you are fortunate enough to be admitted into both Duke and Northwestern. 