I was pretty much in that position going into college too... absolutely fascinated by science, yet interested in law and politics.
First of all, if you are thinking you might want to pursue a career in medicine, then follow that main curriculum. A person with only pre-med classes can go to law school; a person with only pre-law classes can't go to medical school. Better to be safe, in my opinion.
That said, there's advantages to doing a science curriculum anyway. I was a biology major (though never wanted to go to med-school) and I've found that the lab classes and the technical training more than adequately prepared me to think logically, a skill required for the LSAT and for law. Of course, the down side to this is that the curriculum may be harder, thus possibly lowering your GPA (organic chemistry did heavy damage to mine). And nothing stops you from taking philosophy or political science classes as your electives if you are interested in that area as well.
I have to emphasize though that if you really want to keep both options open, then you should pursue things on the side that shows your interest in law or public policy. I volunteered for city councilmembers and got involved in politics, in addition to doing all the biology stuff. That way, when applying for law school or looking for jobs, I can show that I made a gradual decision to attend law school based on interests I've always held, instead of looking like I made an impulsive decision and sudden switch to go to law school because I didn't know where else to go.
It's good advice that you should major in something you like and then worry about law school later... I'll echo that sentiment. But it doesn't have to be one or the other. As I said, consider pursuing a medical-friendly curriculum and law related extracurriculars.