Given that this is what they do day in and day out and that the successful ones are by all metrics very good at it, it seems totally reasonable to assume that one could trivially be turned from manipulating people into buying stuff to manipulating people to voting a certain way or holding certain opinions.
One person one vote is the guiding principle of democracy and, yes, it assumes that no person is able to actively hijack someone else's vote for their own gain. We have systems in place to prevent voter fraud, and I think that we should have systems in place to prevent systematic individual targeting of individuals for algorithmic manipulation as well.
What we don't need is a law that specifically targets foreign companies doing it. Our homegrown manipulators are just as dangerous in their own ways.