Trump has warned that the election process is "rigged," and if he loses, it will mean the election was stolen from him.
How does Trump think election rigging would actually occur?
We already know that voter impersonation (allowing one person to vote multiple times), while possible, is just about non-existent. A recent study found 31 incidents of voter impersonation out of a billion votes cast between 2000 and 2014. Not exactly enough to cause a Trump loss.
Perhaps Trump means that there would be ballot box stuffing, or fraud in the count, or his votes switched to Hillary Clinton. Widespread vote rigging of this sort, however, is flat out impossible because the U.S. does not have a unified national election system.
We have a massively decentralized and locally controlled hodgepodge of election systems, with roughly 9,000 separate city and county election jurisdictions. Every state has its own rules about how voters are registered, how voter eligibility is established at the polls (photo ID, signature comparison, personal knowledge, etc.), and how the vote is tabulated (in the polling station, at some central location). The voting process itself (paper ballot, punch card, electronic of various types) varies even within a state.
Maybe Trump means that a vast conspiracy covering hundreds of polling stations will occur. But that is impossible given the way elections are monitored.
Party and candidate observers (good government watchdogs too) are allowed by law to observe and report on the voting and counting process. The Republican and Democratic parties are already prepared to deploy observers come November. Third parties may also field observers. In a presidential election year, all Congressional Representatives, one third of the Senate, and a wide variety of state and local officials are on the ballot. Every candidate has a powerful interest in using observers to guard against shenanigans on the part of opponents.
Observers typically have a copy of the voter list for the polling place, and check off voters as they arrive. This guards against duplicate voting. If the party or candidate has already identified likely supporters, workers can spend the afternoon chasing down "their" missing voters, perhaps driving them to the polls or offering to babysit.
Observers keep a close count of the total number of people -- both for and against their party -- who turn out to vote. At the end of the day, when votes are counted in the polling station, with all observers watching like hawks, the grand total of counted ballots has to be reconciled against the number of issued ballots and people who came to vote. This plethora of observers is very different from what I saw in Afghanistan, where a complete absence of observers (and voters) in insecure rural areas allowed massive ballot box stuffing by election officials.
As the counts for each office are announced, observers immediately phone results in to their respective headquarters, which aggregate totals to produce city, county or statewide results. (For the Presidential race, state law determines how the statewide vote translates into votes in the Electoral College.) If the official totals do not match observer totals, candidates immediately demand an investigation and use social media to complain. Preventing election rigging is a very local process with a lot of checks and balances.
Given how American elections work, which Trump undoubtedly understands, why is he pushing a false narrative about massive fraud? As we used to say in Chicago, he is "laying a mattress" in case he loses. However, deliberately undermining faith in our perfectly serviceable decentralized electoral processes in order to save face is hardly showing the concern for this country and its institutions that we should expect from a president.
The author has worked and done fraud investigation in Chicago, the Balkans and Afghanistan.