Juan M. Merchan, the judge in Donald J. Trump’s criminal trial in Manhattan, has again held Trump in contempt for violating a gag order banning attacks on witnesses and jurors, and is threatening to jail the former president if he persists. “It appears that the $1,000 fines are not serving as a deterrent,” Merchan said yesterday. “Going forward, this court will have to consider a jail sanction.” Judge Merchan criticized Trump for saying that the jury was “picked so quickly” and made up of “95 percent Democrats.” Trump “not only called into question the integrity, and therefore the legitimacy o…