The Democrats must defeat "the most dangerous president in the modern history of this country," Sanders says.
The days of going to a polling station may be drawing to an end.
The move clears the path for Joe Biden to win the nomination and face Trump in November.
Sanders lost to Joe Biden in the three state primaries that were held Tuesday.
The Biden surge continues.
In a debate dominated by COVID-19, Biden made news with his pledge, while Bernie Sanders equivocated.
It's a make-or-break night for Sanders as Biden tries to build on his momentum from Super Tuesday.