SIMON LEWIS and JOSEPH TANFANI  FOR months, President Donald Trump has tried to convince Americans that the November 3 election will be "rigged," claiming without evidence that mail voting will open the door to mass cheating. "The greatest Election Fraud in our history is about to happen," Trump wrote on Twitter on August 23. In making these claims, Trump has seized upon the idea that U.S. elections are vulnerable to rampant fraud. That once-fringe theory has become a staple of Republican politics, due largely to the efforts of a small network of lawyers who have promoted it for two decades,…