Fully rational or merely fully informed? That is, a rational voter must first be an
informed voter.
But, there is a machine dedicated to disinformation. In the aftermath of Citizens United, we are seeing the rise of dark money to the point where billion dollar campaigns are imminent. Combine that with the consolidation of media, the fall of investigative journalism, and a 24 hour news cycle which has devolved news into infotainment, and it is little wonder that the populace is woefully ignorant. Actually, ignorant is too generous. They are actively misinformed, in that they think they know.
So, I would say no, the current system makes it all but impossible for voters to make rational choices, or even to identify what the real choices are.