Quinnipiac's poll of today has Trump at 27, Rubio at 17 and Carson at 16. Not only does Rubio have a shot, it's increasingly looking like Trump or him, and if they manage to narrow things down to two fast enough, I think we all agree the establishment candidate will take it in the end. For all that Trump has been leading for months and keeps defying the expectations of his impending fall, I think the fundamental strategic point is the same it always was: he's not going to win unless the establishment and the evangelicals remain deeply split all the way to the convention.
Trump or Carson can't compete in the general election. Bush, Christie or Kasich could compete, but they don't look like they can make it there. Rubio could do both - I would still expect Clinton to win in the end, but he'd make it close and all sorts of things could give him the edge. In any case, a close race in the presidential means no Senate majority for the Dems, no House, and if they don't take them in 2016, they won't in 2018 either.