Former President Donald J. Trump, with a deadline fast approaching to secure a roughly half-billion-dollar bond in his civil fraud case in New York or risk seizure of his assets and flagship properties, sent an email on Saturday morning to his campaign’s supporters.The subject line — “Keep your filthy hands off Trump Tower” — was repeated at the start of the email in bold, italics and all caps, even as the message was clearly intended not for his backers but for New York’s attorney general, Letitia James, who brought the case.Mr. Trump told his supporters that Ms. James…