I have one question - “Why?”
The big feeder into the federal government’s science policy jobs are the AAAS science policy jobs. I don’t know if a PhD is required, but the vast majority of successful applicants have one. Further, many discover that big-P Policy is made via the democratic process, and not by Washington technocrats. The best one can hope for is small-p policy, and someone good at their job is good at meshing the very general policy objectives of the politicals to the nuts and bolts of actually running a government. This is by no means entry-level.
Now, if your plan is to study this for a couple of years and then take a relatively unrelated job, great. Lots of people do this. But if you want to work in that field, after 4 years of school you will not be creating policy. It would be more likely you would be working on compliance for people who want to build something somewhere.