Per a 2020 Real Clear Politics article, "Between 2012 and 2018, 28.3 million mail-in ballots remain unaccounted for, according to data from the federal Election Assistance Commission."
That figure amounts to "nearly one in five of all absentee ballots and ballots mailed to voters residing in states that do elections exclusively by mail," the article says.
Further, "States and local authorities simply have no idea what happened to these ballots since they were mailed – and the figure of 28 million missing ballots is likely even higher because some areas in the country, notably Chicago, did not respond to the federal agency’s survey questions. This figure does not include ballots that were spoiled, undeliverable, or came back for any reason."
According to a 2020 Heritage Foundation article, "The U.S. Election Assistance Commission says that in the last four federal elections, 2.7 million mail-in ballots were misdelivered and 1.3 million were rejected by election officials."
Using the “largest individual-level dataset ever assembled to study voter participation,” a paper published by the National Bureau of Economic Research in 2019 found that voter ID laws have “no negative effect on registration or turnout, overall or for any group defined by race, gender, age, or party affiliation.”
A study published in Electoral Studies in 2014 found that the number of non-citizen voters in the 2008 election ranged “from just over 38,000 at the very minimum to nearly 2.8 million at the maximum.”
The study's best estimate is "associated with 1.2 million non-citizen votes cast in 2008."
Other noteworthy findings:
A study published in the International Journal of Public Opinion Research in 2020 examined "precisely how poll results affect voters in an online experiment" using "an experimental design in which votes have real political consequences and which closely mimics real political polling and voting."
The study found that "seeing pre-election polls increased votes for majority options by 7%."