Scenario 2 might just deliver Congress for us if voting patterns are even somewhat similar to 2012. We need somewhere between 7% - 8% win in the House PV to be competitive for a majority, and the higher we go the more likely it is that we break the GOP hold on the chamber. The days of mass split ticket voting are gone (for now) and there will be no 1996 scenario in this political climate.
That said, scenario 1 seems to be the most plausible outcome based on the current data. Scenario 2 seems like the best-case win, and even with a continuation of a Trump implosion, it seems more like to get a PV between S1 and S2.
I won't really even entertain scenario 3 right now. Not until Trump is discovered to be a serial killer with 300+ bodies on his hands, or running a human trafficking ring, or... whatever.
As a Democrat, I'm downright salivating at the thought of something like Scenario 3 (and even Nate himself said ND and NE-AL could flip as well in that sort of landslide). That's the kind of result that would bring about catastrophic results down-ballot. Republicans would be destroyed in a way we haven't seen in at least a generation. It's hard to see right now what would get us that result, as much as I might like to see it. Most likely, it'd take a full-blown public split between Trump and the RNC (i.e. the RNC tries a 1996-style strategy and Trump retaliates with proverbial nuclear force), as in a true civil war in the party right in the middle of a general election.
If the election were held today, I think Scenario 1 is practically spot-on. I think Democrats definitely take the Senate, but I'm not sure an eight-point Hillary victory if enough for Democrats to win back the House. I think it'd be close, maybe a 1/3 chance. The reason I'm more bullish on Democratic chances in the House with a sizable Hillary victory than some is that I think it coincides with a Republican suburban collapse. A big enough win in the suburbs will break down the gerrymanders. For example, if Hillary is indeed up by 40 points or even 2-1 in the Philly suburbs, PA-08 is long gone, PA-07 isn't far behind, and even PA-06 could be in jeopardy. I know the latter two aren't on the radar, but I don't think those kind of margins at the top of the ticket are survivable.
I think the race could evolve into Scenario 2 and some national and state polls do suggest that. I don't think we're there yet, but if things continue as they do, that could be the trajectory. At most, we've probably hit somewhere between Scenario 1 and 2. As I mentioned before, I think that's the point at which the Republican House falling becomes more likely than not. A full-blown Scenario 2 probably gives Democrats both Houses of Congress with some seats to spare. If Hillary Clinton wants to not only win, but actually try to pass some big legislation, she needs to fight hard for Scenario 2.
