To answer the OP’s question: it is possible to rig a national election because the poeple do not elect Presidents, electors do. Generally electors vote for the person the voters choose, but because most states have a “winner takes all” outcome, if you win the state you win ALL of the state’s electors.
The number of electors each state has is based on population, so populous states like Texas, California and New York have many electors. This will be important later.
You cannot rig an election where the other guy has a huge advantage in terms of likely voters, and there’s no point in rigging an election where your guy has a huge advantage. The place to put all your vote rigging efforts is states where things are fairly close and a few hundred or thousand votes can make the difference. Places like Florida in 2000 and Ohio in 2004, both big states with nearly equal number of potential Democratic and Republican voters. (Now you see the importance of big states – just one can swing a national election, lets you focus your efforts nicely.)
It is of course illegal to directly prevent people from voting because of race, religion, etc., but there are ways around this. For example, in Florida they purged the voter rolls of all felons in 1998 and 1999, and also of all people with names similar to felons. This was completely legal, at least as far as the convicted felons go, since Florida disenfranchises convicted felons. (But the people who had names like felons – completely illegal of course.) And the thing is, felons tend to be disproportionately black. And blacks tend to vote Democratic. So the people with names like convicted felons who got disenfranchised were mostly Democratic voters. There were thousands of them. Bush won the election by 537 votes.
You can also get a friendly corporate CEO to provide you with hackable voting machines, as has been alleged in the case of Diebold, whose CEO actually publicly promised to deliver Ohio to the Republican Party in 2004. (There have been many allegations that Diebold machines are hackable, though to my knowledge no hack has been proven well enough to go to a court of law. The point of hackable machines, however, is that it’s impossible to trace the hacking on the models that don’t have paper trails.)
There are many other techniques for rigging elections and disenfranchising voters – in the South it was something of an art during the Jim Crow era. Hillary could use any of them. But mostly its the Republicans who are motivated to use them, as their voter base is rapidly shrinking. If they didn’t have the evangelicals, they’d be dead in the water.