He's easily the biggest dark horse on the Democratic side of the aisle. There were a few rumblings right after the election about a potential candidacy from him but not anymore. He's almost completely out of the discussion as of right now and that's a good thing for him this early on.
If I'm not mistaken he stated at one point something along the lines of "Everybody says they aren't running until they run." I think he'll jump into 2020.
I think the reason the rumblings have subsided is because of his re-election campaign. Like Morden said, if he's re-elected (likely) and a Democrat wins the governorship (dicey), then talk of Brown will increase.
