Part of the reason for this is that Republican leaders have been sounding the alarm on "voter fraud" for years in an attempt to drum up support for restrictive voter ID laws -- laws that tend to negatively impact Democratic voters more than Republican ones.
But federal courts have lately been pushing back on these measures. Most strikingly, in late July a federal appeals found that North Carolina Republicans attempted to systematically disenfranchise black, Democratic-leaning voters "with almost surgical precision" in passing that state's voter ID law.
The court found that Republican officials requested data on racial differences in voting patterns across the state, including on the 300,000 to 600,000 voters who lacked ID issued by the state Department of Motor Vehicles. Then, "with race data in hand, the legislature amended the bill to exclude many of the alternative photo IDs used by African Americans," the judges wrote. "The bill retained only the kinds of IDs that white North Carolinians were more likely to possess."
Earlier in the summer, a conservative federal appeals court struck down a Texas voter ID law on the grounds that, while the law wasn't discriminatory in intent, it became discriminatory in practice as it had a far greater impact on minority, Democratic-leaning voters. About could have affected 600,000 people lacked the necessary identification, disproportionately minorities.
More recently, a federal judge in Florida ruled against the state's "obscene" refusal to allow voters to fix signature mismatch issues on mail-in ballots. These mismatches were rare, happening on fewer than 1 percent of all mail-in ballots. They often occurred when voters' signatures changed over time, when voters used both cursive and block letters to sign forms, or when signatures simply weren't legible.
An analysis conducted by political scientist Daniel Smith of the University of Florida found that Democrats were considerably more likely to have their votes invalidated due to signature mismatch issues than Republican voters were.
Returning to Donald Trump, the candidate's arguments against "voter fraud" appear to be part and parcel with his recent accusations that the presidential vote will somehow be "rigged." But as the cases above illustrate, Republican lawmakers have been attempting to systematically alter the composition of the electorate in their favor for years.