There are different levels of "gaming".
Both parties manipulate the primary process, to make sure that "their" candidates are the ones presented to the general public.
Both parties manipulate the rules and regulations about who can vote where, although Democratic manipulation tends to be about opening up the process, while Republican manipulation is all about preventing people from voting, and with gerrymandering, trying to negate the votes they can't stop. The Republican activities are thus inherently much more distasteful to anyone who believes in democracy.
It is quite possible, and in my view, quite likely, that some elections are fixed by changing the vote counts - actual election rigging. Not all over the place, but in some locations. That is obviously not acceptable to honest people, and it has the capability to change the overall result even in national elections, because with the flaky winner-take-all set-up, changing just a few local counts might swing a state, and just one or two states different might be enough to change the result. Witness Florida in 2000. Despite various attempts to say that Bush would have won a recount if one had been allowed, that isn't clear, and in fact when you got into it, didn't seem all that plausible. (If it were really so clear, the Republicans wouldn't have been so desperate to prevent a recount.)
Why do I think that some places are rigging elections? I remember back in the 90s, there were elections taking place elsewhere in the world. I don't remember where, exactly - I think maybe Africa, but it may have been ex-Soviet states, or even all of the above. In any case, it was countries with no great record of free democratic elections. And there were monitors in place from other countries (probably from the UN, thinking about it), trying to judge whether the elections really were fair. That seems like a fairly hopeless task, but we were assured that they could do a good job, because modern exit-polls are so good and accurate that they are virtually always correct. Any deviation of serious size of the official result from the predicted (via exit poll) result could be taken as a very strong indication that the votes weren't counted correctly. And US election results from the 80s and 90s were touted as examples of how good the correlation was. Guess what - US elections from 2000 onwards haven't been such paragons. And these days nobody talks about how good exit polls are.
There is also the voting machine bullshit. There is no real reason to have voting machines at all - in Britain, people do just fine with a piece of paper and a pencil (there's no reason to make the slip as complicated as Americans do). But if you do want to make a voting machine, it should print a marked-up paper ballot as a receipt, so that the voter has a hard-copy verification that his vote has been correctly recorded. These receipts get put into a ballot box on the way out, so that if there is any doubt, they can be counted as a manual recount of the election. This is just an obvious piece of process design, that can't be difficult to implement technically (a modern grocery store cash register does technically more difficult stuff). And yet some places (mostly Republican-controlled places, for some reason) prefer voting machines that provide no record and have no facility to do any sort of recount. Some of the precincts with the most surprising (per exit poll) results use voting machines of dubious provenance. In Florida 2000, this was certainly true. There was a big fuss about some new voting machines that were obviously not to a proper standard, and much Republican virtuousness was touted because their results seemed to be valid. No attention was paid to other precincts that used older machines with a different technology (I think you marked up a piece of paper and the machine scanned it). Those machines had been in place for a number of elections - and their results were quite dubious. But nobody paid any attention, only looking at the shiny new things.
I don't know if there was any of this going on during Obama's elections, but I think he was winning by large enough margins that it didn't matter - the Republicans couldn't rig enough to change the result without it being obvious. The 2016 election (no matter who are the candidates) will be closer.