(1) The tweet is a lie. Only approx 1 million Muslim voters in a General Election. Not all Labour. Only 3 million ethnic minority voters in total: one third are Muslim. This is a good reason to delete the tweet.
Another good reason to delete the tweet?
Best recent evidence is the CST study. *Have you read it?* Majority of Muslim respondents did not accept any of the anti-semitic tropes. But larger minority than general population
Groups like LeaveEU and Breitbart (if serious about anti-semitism) would be tackling prejudice among your base, not stoking it.