In the first scenario I have Clinton beating Trump 47.0%-41.0%. This is a slightly wider margin than now, especially after the Monmouth poll drops off the RCP average. I'm making the assumption that the 3rd party candidates take 12% of the vote. That's roughly where they are now without factoring in undecided voters. If we were to project undecided voters voting evenly for the candidates the third parties would do better than 12%. In most elections, however, they don't exceed 5%.
Next I wanted to figure out congressional retention. Some people leave down ballot races blank. In 2012 Republicans got a higher percentage of Mitt Romney voters than Democrats got from Barack Obama voters. President Obama won by 5 million votes. Congressional Democrats won by 1.4 million. Then I allocated Johnson-Stein voters, giving Republicans a 9 point edge with them. I reason that our assumption is that voters who'd normally vote Republican for President won't vote Trump. So the retention is higher, as it was in 2012, as well as the percentage of third party voters. This gave Democrats a 2.4% win in the congressional elections, higher than their 1.2% win in 2012. In 2012 that 1.2% win got them 201 seats. Based on 2012 margins I see a 2.4% win getting them 205 seats. That doesn't sound like a lot, but it's a 17 seat gain. The prognosticators are saying Democrats will gain 10-15. So even in what I view as a likely scenario with several Republican assumptions the GOP still suffers serious losses. Just not enough to lose the House.
So what'll it take for Democrats to win the House. With an 8.8% Clinton win, voter retention not as good, and a more equitable third party reallocation, we end up with a Democratic 7.2% congressional win. I think that'd get Democrats a majority.
The assumptions here favor Democrats but, so I conclude that a Clinton win in the 9% range wouldn't definitely cause the House to flip but it'd be in danger of flipping. Cost posits that Trump would need to get at least 44% of the two party vote. In my example here Trump has 45% and I think there's a better than average chance the House would flip. If Trump loses the two party vote 56%-44% the House would be in serious danger for Republicans. The good news for the GOP is that the RCP average has never had him below 45.3% of the two party vote. Trump is doing better than that now. Of course he could still drop.