Bernie Sanders being Jewish has absolutely no bearing on his fitness to serve as president. His political views are what determine his fitness. Mentioning his socialism is fine, but the precinct captain shouldn't be bringing up Sanders' being Jewish at all. It was uncalled for, and I hope that the voters will replace the captain with someone more suitable at the next election.
When you take on a position of public service it is your job to carry out your duties in a decent fashion. In the area of public service you cannot have a bias, it is your duty to be objective. Now sometimes people get passionate but in order to be a successful public official you must check your feelings at the door.
The Iowa caucus runs on the premise that the precinct captains are all native Iowans. If Hillary Clinton's campaign is using out-of-state staffers as precinct captains to try to influence the outcome of the election then the captain was certainly out of line. Hillary Clinton did not want Obama to do this in 2008 and she should not either.
Even if not revealed as a paid member of Hillary Clinton's staff is it almost inarguable that not allowing Bernie Sanders to speak, and using racially charged language, is despicable and very much out of line. Especially with so many eyes on the election race this behaviour can not be condoned.