And now, more polling data suggests this is fertile territory for Trump to plant seeds of doubt in case he loses. Trump backers this year, it turns out, are uniquely skeptical of the vote-counting process.
In new Pew poll, just 11 percent of Trump backers say they are "very" confident that the votes in the 2016 election will be counted accurately. Another 37 percent are "somewhat" confident, and half of the GOP nominee's supporters are either "not too" (31 percent) or "not at all" (19 percent) confident.
In contrast, 49 percent of Clinton supporters are "very" confident, and only 20 percent are either "not too" or "not at all" confident.
What's especially interesting about the Pew numbers is that they indicate the shoe was once on the other foot.
Back in 2004, it was Democratic voters who were more concerned about inaccurate vote counts. When Pew asked how confident they were that their own votes would be accurately counted — a more personal version of the previous question — 48 percent of Democrats said they were "very" confident, while 75 percent of Republicans were.
This was very likely the lingering effects of a contested 2000 presidential election — Florida, "hanging chads" and all that — which wound up going Republicans' way.
But even then, Democratic voters were more confident in the vote-counting process than Republicans are now. Today, even without a similar recent controversy for Republicans to point to, those numbers are 67 percent for Democrats to just 38 percent for Republicans.
So, to reiterate, even after a 2000 election in which many Democrats thought Florida election officials essentially miscounted votes and handed the presidency to George W. Bush, Democratic voters were still more confident about the vote-counting process than Trump voters are today.
Now, this doesn't mean that the vast majority of Trump backers are expecting a totally "rigged" election. It's possible that they think the inaccuracies would be too small to matter — and that the inaccuracies might not be deliberate. Miscounting votes is not the same thing as large-scale voter fraud.
But as I wrote earlier this week, other data does indicate that many Republicans believe there could be rather large-scale voter fraud. A majority of Wisconsin Republicans — 54 percent — said in 2014 that fraud of one type or another moves thousands of votes in the state in every election, despite a lack of evidence to back up that theory.
And with the 2016 GOP nominee pushing that idea hard, it seems quite possible to catch on.