Yikes. The constant appraising of "normal" Republicans and effort to expunge the GOP of any guilt in creating the monster that is Trump has been one of the worst parts of this campaign. It doesn't seem to sway Republican voters, and it probably depresses the enthusiasm of Dem voters.
These seem like claims like "If Bernie Doesn't Drop Out This Instant Trump Will Permanently Win And Actually We've Already Lost Because Bernie Is Mean To The DNC!!!!!" -- I guess pluasible, but not actually backed up by evidence and mostly the product of personal anxiety about the state of the race and the nation. I absolutely expect that the skillful campaign Hillary ran from June to the post-convention helped encourage soft-Republicans to abstain and built up good-will from Independents. And we've already seen that to the extent that there are a category of voters (say, the unexpected Johnson numbers) who are depressed about Hillary but won't vote for Trump, it seems these voters are motivated by a strong devotion to sincerity--in other words, that I think they have written Hillary off irreperably. Most of them also won't vote.
Plenty of Democratic candidates are going to come out and say "I'm a progressive, and I believe we are a country of progressives." No Democratic candidate is going to come out and say "I'm a progressive, and fuck Republicans, they're evil and I hate them." If you're hoping for the latter, I'm sorry but you're being myopic.
Only the Democrats would so expertly fumble at a golden opportunity to tarnish their political rivals name for a generation. There isn't a chance in hell the GOP would have been so kind had the Dems nominated the left wing equivalent of Trump.
I think directly appealing to brinksmanship is pretty immature. It's a bad thing that Republicans immediately responded to Obama by blocking everything he did and saying they'll spend all their capital blowing up government and making him a one term politician. But they did it because they cultivated the idea that they were right, Democrats were wrong, and winning was the most important thing. If they had adopted a flexible approach, the country would have benefitted substantially from it. I am to your left politically, I am not saying this because I believe in some third way shit. I am saying this because at some point the rubber needs to hit the road to govern, and whether it's in campaigns or governing a recognition that we can disagree, even strenuously, and view many of the other side's ideas as odious, but in the end we need to work together. Trump's rise is a product of Republicans cultivating a view in their base that you need to either fall in lockstep with policy ideas or you should be obliterated. The reason "cuckservative" took off, the reason the tea party took off, is because an increasing number of people think "No Compromises" is a good way to run the country. I disagree. Parties should capitalize on victories, but capitalizing on victories is still possible within magnanimity and extending a hand to work with others.
Obama's approach to dealing with the GOP has been the correct one. Offer an olive branch, work with them, and if you exhaust all other options, do what you can to make progress regardless, but always make it easier for them to work with you and against you. That the GOP appreciate this doesn't make Obama wrong for doing it.
Also I reject the idea that there is a "left wing equivalent of Trump", unless you mean some idiot playing hacky sack outside the student commons at Berkeley.