We have reached a point we never thought possible. Donald Trump is not only going to be the Republican nominee for President, but he will have wrapped it up long before the convention. There will be no contesting, there will be no infighting. From today until July, Donald Trump is the nominee, and he can turn to the general election without hesitation.
That is why it is incumbent on Bernie Sanders to do one of two things; 1) Drop out of the race, or 2) Justify his path to victory.
Bernie said not long ago that he will do "everything in my power" to make sure a Republican doesn't win the White House. That statement, by definition, includes his ability to end the Democratic primary season.
While we can sit here and say that Donald Trump has no chance of ever winning the election in November, we also said the same thing about the primary. The mere fact that it is possible means that we, as Democrats, should be doing everything we can to destroy his chances beginning today. That is not possible, however, while we are still embroiled in an active primary. Hillary cannot move on to raising money and fully organizing a general election campaign while Bernie is still in the race. It would be viewed as an insult by many of his vocal supporters, and party unity is too important to allow that.
That leaves Bernie in firm control of what happens next. For the sake of the party not giving Donald Trump a one or two month head start on his never-ending string of attacks against Hillary, the entire party needs to be able to respond in lockstep, without fighting a two-pronged war. Every attack leveled by Bernie at this point not only reinforces the toxic image Trump will try to portray of her, but it distracts her from being able to fight back appropriately. Though he doesn't mean it, Bernie is opening a flank for Trump to attack.
This would be moot if we were still in the midst of a close race. Bernie has lost, though. The math against him is daunting, which even he admits when he and his advisors make the case that their only path to winning is to have the establishment rig the process to steal the nomination away from the choice of the voters. By actively promoting such an undemocratic usurping of the process, Bernie is engaged in a bad faith attempt to wrest away the nomination, de-ligitimizing Hillary, and giving yet more ammunition to Trump.
It is incumbent on Bernie, if he wants to continue on in this race, to justify how he can win through democratic means. If he cannot make a case, his continued presence does not help the party by pushing it to the left, it does real damage by giving Trump a head start. Even now, as Bernie is accusing Hillary and the DNC of money laundering, we can see how easily Trump will take Bernie's own words and use them for his own benefit.
The time has passed for Bernie. With no path to the nomination, he is holding back the party from doing its job and beginning the fight for the White House. Bernie's entire campaign has been built on attacking the Democratic party as an institution, but as a man running to be its standard-bearer, he must realize that the right thing to do is to show leadership and bow out, so there is a future for him to influence.