It’s shaping up to be a bad night for Sanders, given that it looks like he just lost Michigan. That was a state that Hillary really struggled with (and where Sanders did well) last time. If Sanders can’t do well in the Rust Belt, a region shafted for decades by Establishment Democrats like Biden, it’s close to game over for Sanders.
Close, but not quite. Biden has yet to be tested in a one-on-one debate, and given that verbal skills are one of Biden’s chief weaknesses, he needs to be. The next debate is scheduled for the 15th. If Biden at least holds his own in that debate, and goes on to continue winning primaries, then it will be time for Sanders to consider conceding.
At that point, Biden could really help put pressure on Sanders, by sending a signal that he realizes the Democratic Party is a big tent. This doesn’t mean adopting Sanders’ platform (Biden, not Sanders, will have won, after all), but it does mean emphasizing at least some progressive concerns, to a greater degree than he already has. Choosing a running mate or some cabinet posts could well suffice. This is something that moderates have had a great deal of difficulty acknowledging in the past, persistently trying to ignore or belittle those to their left. Witness Hillary Clinton’s tone deaf VP choice last election.
If, however, Biden then sends that signal, then it’s definitely time for Sanders to drop out. But only then.