There are any number of problems with this, again starting with the fact that the frequency of in-person voter fraud in elections is
lower than getting five numbers right in the Powerball. But there's a potentially bigger legal problem
noted by election law expert Rick Hasen of the University of California at Irvine: Trump's unnecessary effort could be violating a prohibition against voter intimidation that applies to the Republican Party.
In 1981, the Republican Party rolled out a voter-integrity effort in New Jersey that mirrors what Trump demanded in Altoona. As described in
a legal ruling about the prohibition:
The RNC allegedly created a voter challenge list by mailing sample ballots to individuals in precincts with a high percentage of racial or ethnic minority registered voters and, then, including individuals whose postcards were returned as undeliverable on a list of voters to challenge at the polls. The RNC also allegedly enlisted the help of off-duty sheriffs and police officers to intimidate voters by standing at polling places in minority precincts during voting with “National Ballot Security Task Force” armbands. Some of the officers allegedly wore firearms in a visible manner.
(Trump in Altoona: "We have to call up law enforcement. And we have to have the sheriffs and the police chiefs and everybody watching. ... The only way they can beat it in my opinion — and I mean this 100 percent — if in certain sections of the state they cheat, okay?")
The Democrats sued, and in 1982, the two parties agreed to a system under which the Republican National Committee agreed to refrain from a number of tactics that could be used to intimidate voters. That consent decree, as it is called, has been modified a number of times, often in response to efforts to challenge the ability of Democratic voters to vote, occasionally targeting black voters specifically.