Two days before the Michigan Democratic primary, speakers at a rally on Sunday in Dearborn, Mich., urged voters to withhold their support from President Biden over his policy on the war in Gaza — and said that only Mr. Biden and Democrats who support his Israel policies would be to blame if the protest vote helped former President Donald J. Trump win in November.“You all know Trump is an existential threat to our democracy,” said one of the speakers, Representative Rashida Tlaib, Democrat of Michigan, “and President Biden is risking another Trump term over his support for the most…