18% is a big number among likely voters, and I have never seen such a large gap before, but beyond that, of more interest to my point, is the gap between all voters and likely voters, with the GOP doing much worse among the latter. Have you ever seen such a large GOP deficit between the two? I haven't.
I should have pinged you to my reply. In any event, the dynamics in Senate races is quite a bit different. Personalities matter more. Incumbency matters more, plus or minus. The odd thing, is that the GOP seems at the moment to be doing somewhat better in the Senate, while tanking for the House. Yes, that might change, and the two converge, and that certainly might be more likely than not, but it is not true now.