I myself have supported Bernie for president since day 1 including with a number of modest donations, and was among those hoping even before that that he would run AND TO RUN IN THE DEMOCRATIC PRIMARIES and not as a third party splinter candidate. I hope and expect that he will continue to campaign until June, amassing as much support and as many delegates as possible, and building up that crucial network for future political organizing (which as the 'progressive opposition should start soon after the Nov election no matter who wins), and that if Hillary is the nominee, and it now seems likely, I hope and expect that he will actively support her and Democrats in a wide range of key Congressional/Senatorial races in the GE. In part, his support for Democrats in the national election if he does not get the nomination will make it that much easier to be able, after November, to organize the (relatively) progressive wing of the Democratic Party, from the grass roots. Some have insultingly compared Bernie and such possibilities to the teabaggers of the right in terms of substance ('uncompromising') although Bernie is spot on while the teabaggers are completely full of crap -- however, if progressive Democrats could get to the point organizationally reached by the Tea Party & the ideological right in this country that would be an achievement. Hopefully the relatively progressive wing of the Democratic Party will be MORE successful over time within the Democratic Party than (so far) the teabaggers have been in the GOP, and will triumph within the Party and the country as a whole over the various political forces, from neoliberalism to neanderthal reactionary politics, to its right.
I know however, that there are MANY supporters of Bernie who feel the way expressed in the OP, and especially want to convince those that are in swing states not to stay home, and to make SURE that neither Trump (nor Cruz) ever occupy the White House. The notion that a resounding defeat of the mainstream of the Party will help progressives didn't work in 1980, or 84, or 88. We only got the Party moving further to the right under Bill Clinton. And since then, including Obama, who has turned out as president to be far more centrist than many of his supporters from day 1 in 2007, including myself, had hoped, the Democratic Party has remained in the grip of various shades of neoliberalism, where it remains.
Bernie's delegates can fight to win a number of platform concessions, on particular points often with a large number of supporters among Hillary's delegates. And after November BOTH within AND outside the Democratic Party, hopefully Bernie himself and his network, his lists of donors and his networks in states across the country from the primary campaign, can be the core of an opposition.
I would add that although no serious multi-issue progressive opposition arose within the party (or outside) in any sustained way under either Bill Clinton or Barack Obama -- the most important so far being the Black Lives Matter movement, which unlike Occupy has continuous long-term prospects -- having a neoliberal Democrat in the White House would provide a clearer opportunity for this kind of mobilization ESPECIALLY WITHIN the Democratic Party than having someone like Trump. In the latter case, neoliberals and progressives would be together in opposition, and usually the neoliberals would win out, as in turned out there were no serious alternatives to neoliberalism at the end of W's disastrous presidency
But how can Bernie supporters like myself who urgently call on fellow Sanderistas NOT to stay home or (in swing states at least) vote Green or whatever, if there is any chance of a Republican getting the White House? By effectively limiting DU to those who support the nominee, the others will flee or silence themselves. I have posted numerous times on Bob Reich's facebook conversations and they are ABSOLUTELY not the same thing, though they have some value
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