A few things:
There's no prereqs for law school. If that's what you want to do after undergrad, jumping from History to Political Science is really not going to make any impact on either your admission prospect nor your ability to do well (fyi: the majors who, on average, perform the best on the LSAT are Math/Comp Sci)
However, if you don't see a future for yourself in History then there's no good reason why you shouldn't switch (barring restrictions from your institution.) There are people who completed their entire undergrad doing History, earned their degree in it, then switched over to Poli Sci for grad school; I think you'll be fine.