The ballots were among 1,875 pieces of mail that were recovered.
The YouGov poll indicated just 11% were confident election results would be accepted peacefully.
Trump's campaign insists the president is healthy and will be able to attend the next presidential debate.
Political experts predict that the vice presidential debate will be more civil, due to the candidates' debate style.
In past elections, the vice presidential debates have been little more than a side show, but this time they have taken on added significance in light of the candidates’ ages, as well as Trump’s illness.
With Trump infected with the coronavirus, the Pence-Harris showdown has taken on an unusually pressing quality, given that the White House deputy is just a heartbeat away from the presidency.
Trump calls off stimulus talks, Fed says recovery faster with aid
Pandemic, protests, politics driving US gun sales
Trump defies Covid, polls -- but Biden keeps gaining
Both second lady Karen Pence and Kamala Harris’ husband, Doug Emhoff, have hit the campaign trail this year on behalf of their spouses, as the November election looms.
President Trump said he'd rather see the Senate concentrate on confirming Amy Coney Barrett to the Supreme Court.
US recovery would be stronger, faster with more aid: Fed's Powell
Biden urges pressure on Turkey over Greece tensions
QAnon conspiracies go global in pandemic 'perfect storm'
John Oliver talked about President Donald Trump’s COVID-19 diagnosis and blasted the first family for not wearing masks during the first presidential debate.
Stock markets rebound on Trump health, stimulus hope
The coronavirus cases in Trump's inner circle
President Donald Trump told social media followers to not be afraid of COVID-19, with detractors criticizing the statement as insensitive to those who lost loved ones in the pandemic.
Trump leaves hospital for White House -- removing mask immediately
Polls have been favorable to Joe Biden and Mark Kelly with the election less than a month away.
Pelosi, Mnuchin continue stimulus negotiations
Biden so far has tested negative for coronavirus and says he will debate Trump Oct. 15 as long as scientists say it is safe.
