Lots of people seem to think it's rigged. People who would presumably know.
How about those computers that count your votes? In Georgia, they record your vote on a credit-card shaped device, then wipe it after they transfer the information to the central system. At least, that's allegedly what happens. There's no record of how you voted, and the people transferring the data have nothing to look at that would indicate that the transfer was done fairly. There's also no paper ballot or receipt that could be used for a recount. That doesn't prove the system is correct, but it gives us reason to suspect the system.
In that context, then, here's some sworn testimony from a programmer who says he wrote software to rig elections. There is also a discussion thread at Snopes about him and his testimony.
George Will points out that the IRS scandal is itself proof of official attempts to rig the election against conservatives, by preventing them from organizing or collecting tax-deductible donations, and harassing them with audits.
And of course, as we (and left wing Vox) recently discussed, gerrymandering is the Fire-Breathing Godzilla of vote rigging. My Congressional district is R +27. If I and everyone I know voted Democrat this year, it might fall to R +25. Other districts are just as heavily D +. The same is true of state-level districts.
Of course the vote is rigged, in every way they've figured out how to get away with rigging it. The animating questions should be whether there's anything left that isn't rigged, and what (if anything) we can do about the parts that are. If the answer to that latter question is that we can't do anything that will legally compel the powers that be to back off their vote-rigging, we should start talking about what to do about that.
UPDATE: Another Vox piece demonstrating a proven, actual history of rigging elections -- Jim Crow, of course.
That example is hopeful, in a way, because the Jim Crow system was eventually dismantled (except, arguably, for gerrymandering -- or if you take seriously the complaint that things like Voter ID laws are intimidation efforts). But it's also a telling example for the plausibility of rigging an election. Of course it can be done. It always has been done. It's done everywhere anyone figures out a good way to do it. Often, as in the case of gerrymandering or the IRS scandal, it's done right out in the open.
The question is, what can we do about it?