I think it would be easy actually, even easier than what they did. I think the trick would be to get the numbers of frauds high enough, and wide-spread enough. Makes me wonder how easy it is to get false documentation to make it through our vetting system.
Over the past twenty years or so, perhaps the larger percentage of registrars in this country have quit purging their voter lists, so there is a lot of room for voter fraud. When someone can know that the registered voter is not going to be showing up to find that someone has already voted in his name (such as when they are dead or no longer live in the area), these are potential votes that could be taken by someone, even more easily when there are willing partisans working at the polling booth as well.
If anyone was up to the task of voter fraud, it was not the conservatives imo. The reason the left freaked out early on, and was "up to" anything that would stop Trump was they were scared as his support grew, and grew, and grew.