Unelectability arguments come from conservative anti-Trump pundits and Democrats who don’t want a left-leaning president. Those fearing Republicans insisting Sanders is a “socialist” and equate his policy positions with Soviet-style Communism should remember that Republicans apply such labels and worse toward any Democrat, even in situations where the tactic is ludicrous. Republican attacks are going to be vicious and dishonest whoever is nominated.
Democratic victory comes within reach if Democrats mobilize higher turnout among youth, people of color, and other left-leaning Democrats and independents who stayed home or voted third party in 2016. Sanders has demonstrated he can do this in increasing youth turnout in the primaries and building a multiracial coalition. To win in the general election, Democrats need to beat Republicans in the enthusiasm gap.