Trump tweets about mail-in voting rigging the elections this year
"RIGGED 2020 ELECTION: MILLIONS OF MAIL-IN BALLOTS WILL BE PRINTED BY FOREIGN COUNTRIES, AND OTHERS. IT WILL BE THE SCANDAL OF OUR TIMES!"
He has made mention to this issue several times in the past month but it is starting to gather more traction in the past few weeks, as recent opinion polls have seen support for Trump slip (though you can't really extrapolate much from that at this stage).
Trump's major concern is that mail-in voting could result in potential fraud with ballots possibly being stolen from mailboxes or even counterfeit ballots being printed by foreign powers looking to sway the outcome of the election.
There's now less than five months before the 3 November election so expect things to heat up more and more, day by day. In turn, this will also be more of a factor to markets in general as the clock slowly ticks down as well.