My suspicion is that the polls now and in 2012 WERE undercounting GOP votes ... but that's because they have to rig the polls so that they line up with the rigged election results.
OR, America is dead and full of dumbasses
As for the size of the crowds at the rallies, all those measure is enthusiasm ... an enthusiastic vote for Trump counts just as much as an "oh well I guess I'm voting for Hillary".