We based our notions of who is smart or not in politics mostly based on how they sound when speaking (and of course whether they mostly agree with us).
I got to know our Representative in Ohio a bit and heard several times he was not very bright (he's no longer in office). He did some nice things for our family, so I won't speak ill of him, but he didn't sound very smart when speaking. I met our then Senator a few times, he's now Ohio governor (Dewine), he struck me as reasonably intelligent, and he also did some nice things for our family (like writing a recommendation to law school for one kid). I met John McCain briefly and heard him speak in person, I thought he was reasonably intelligent. He also said some things he knew the crowd wouldn't like hearing.
Anyway, for me, it's facile to claim "X is dumb as a rock" SOLELY because they disagree with your positions politically. I think Liz Warren is probably pretty smart and I agree with her on almost nothing. I think Bernie is dumb as a rock.