Now Mr. Sanders’s campaign is in final discussions with MSNBC to appear on the network for a prime-time town hall before the next round of primary voters go to the polls on Tuesday, according to a person briefed on the discussions.

It would be Mr. Sanders’s first MSNBC town hall in nearly four years. The network had a longstanding invitation, but the senator — whose campaign manager, Faiz Shakir, once dismissed the network’s coverage as “terrible” — had declined the offer until now.

Mr. Sanders’s sudden embrace of MSNBC seemed to signal a newfound need to engage with a broader swath of a Democratic electorate that is rapidly coalescing around his opponent.

And it was an unlikely turn for the cable network just days after one of its longtime stars, the anchor Chris Matthews, was forced to resign.