Under the Electoral College system, as prescribed by the Constitution, the Presidential election is on a state by state basis. Presidential elections are governed by each State and not by the Federal Government. In order to "rig" the Presidential election, it would have to be done by somehow taking over the election process of most of the governments of 50 different States.
Since most of the State governments are in the hands of Republican governors, when Trump complains that the election has been rigged against him, he is, in effect, indicting his own Party.
Since the Democrats have no dog in that fight, they should just butt out and leave the fight to Trump and Priebus.