You probably don't want to alter the votes in the swing states, because there will be a lot of scrutiny on them. But if you alter a couple of county's results

You probably don't want to alter the votes in the swing states, because there will be a lot of scrutiny on them. But if you alter a couple of county's results that were assumed to be "safe" to /just/ swing to the other party, more than likely it won't receive any major attention, but could cancel out the delegates from the other county and get you the desired result. To affect a national election, you'd still need a small army of helpers - "vote smurfs" to inject your votes. Chances are very slim of keeping that quiet.