For months, U.S. President Donald Trump has tried to convince Americans that the Nov. 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 Aug. 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, funded by right-wing foundations.