Sen. Bernard Sanders‘ grassroots activists fear a repeat of 2016 in next year’s Democratic primary — this time, instead of former Secretary of State Hillary Clinton, they see the party establishment rallying around former Vice President Joseph R. Biden Jr. in an effort to deny their candidate the nomination. For hardcore Sanders supporters, Mr. Biden’s early domination of the race smacks of 2016, when the Democratic National Committee attempted to sabotage Mr. Sanders and clear the way to the nomination for Mrs. Clinton. “The mainstream media and the DNC are colluding against the American people. That’s what it feels like....