"Bernie Sanders is definitely being underestimated in Iowa," Democratic Chairman, Poweshiek County.

Why Bernie Sanders Is Tough to Beat

(snip) Bernie Sanders is definitely being underestimated in Iowa,” said John Grennan, the Democratic chairman in Poweshiek County, Iowa.

“Part of his durability is that he has 15 to 20 percent of the caucus who are absolutely committed to voting for him no matter what,” he said. “In a field that’s split between at least 10 major candidates, that 15 to 20 percent counts for a whole heck of a lot.”

There are other factors that have helped Mr. Sanders in Iowa. Because his backers are so loyal, opponents have been unable to penetrate his base, if they have tried to at all. Part of the reason is that Mr. Sanders’s strategy revolves around engaging people who typically don’t participate in the political process, a highly difficult group to target; even the Sanders campaign acknowledges it is a risky strategy. Another factor is sheer resignation: His rivals just don’t see the point in trying to pick off supporters who probably won’t budge

Mr. Sanders has also mostly escaped aggressive attacks from his rivals. Other candidates have focused more on trying to stop Ms. Warren, whom they viewed as a bigger threat. On the airwaves, Mr. Buttigieg, another front-runner, has run television ads that attack Mr. Sanders’s proposals like “Medicare for all” and tuition-free public college but do not name him directly.

Ms. Warren herself has rarely criticized Mr. Sanders. Asked at a recent stop in the blue-collar town of Ottumwa what made her a “better candidate” than Mr. Sanders, she responded tepidly that they had been “friends for a long, long time.

https://www.nytimes.com/2019/12/26/us/politics/bernie-sanders-iowa-democrats.html

I would add something else. The Sanders Campaign welcomes (public) criticism. The more the better.