The most stupid thing is probably flip-floping. But he did something that was pretty hair-brained the other day - he attacked Gingrich for voting against GHWB’s (read my lips) tax increase.
I was going to correct your spelling (it’s “hare-brained”),
but in thinking about it, the overmanaged coiffure of Mitt also makes him “hair-brained”. So-—no correction required.