Mr. Sanders campaigned for Ms. Teachout in her unsuccessful 2016 run in New York’s 19th Congressional District, helping to attract his avid supporters by appearing alongside her at campaign events. Ms. Teachout endorsed Mr. Sanders in his 2016 campaign against Hillary Clinton for the Democratic presidential nomination.

Ms. Teachout does not work for the Sanders campaign, but she has endorsed him.

In her opinion piece, she argued that nominating Mr. Biden would make it harder for Democrats to defeat President Trump.

“It looks like ‘Middle Class’ Joe has perfected the art of taking big contributions, then representing his corporate donors at the cost of middle- and working-class Americans,” Ms. Teachout wrote. “Converting campaign contributions into legislative favors and policy positions isn’t being ‘moderate.’ It is the kind of transactional politics Americans have come to loathe.”

Mr. Sanders told CBS News that he disapproved of his supporters demonizing political opponents online, saying “we need a serious debate in this country on issues.”

“I appeal to my supporters: Please, engage in civil discourse,” he said. “And by the way, we’re not the only campaign that does it. Other people act that way as well. I would appeal to everybody: Have a debate on the issues. We can disagree with each other without being disagreeable, without being hateful. That is not what American politics should be about.”