https://www.wsj.com/articles/mail-in-voting-could-deliver-chaos-115983764
If the 2000 election provoked a constitutional crisis, the 2020 one is flirting with disaster. Debate over voting by mail has focused mostly on the potential for fraud and logistical difficulties. But there are also legal problems with it, which carry the seeds of chaos before Inauguration Day and continuing instability after.
Under federal law, the presidential election must take place on Nov. 3, and the electors chosen on that day must vote on Dec. 14 to select the new president and vice president. These dates can’t be changed without an act of Congress, and the 20th Amendment sets Inauguration Day on Jan. 20.
Article II of the Constitution gives Congress the power to “determine the time of choosing the electors, and the day on which they shall give their votes; which day shall be the same throughout the United States.” Congress has done so by enacting laws mandating that “the electors of President and Vice President shall be appointed, in each State, on the Tuesday next after the first Monday in November,” and that the Electoral College must meet and vote on “the first Monday after the second Wednesday in December.” As the Supreme Court held in Foster v. Love (1997), taken together the relevant constitutional and statutory provisions mandate “holding all elections for Congress and the Presidency on a single day throughout the Union.”
It follows that although state statutes permit the use of certain mail-in ballots sent on or before Election Day, no ballot cast after Nov. 3 is constitutionally valid. That implies that counting unpostmarked mailed ballots that arrive after Election Day would be unconstitutional, as there would be no way to tell if they were cast in time. In addition, the winner of each state’s electoral votes must be determined by Dec. 14, or those votes cannot be cast.
These requirements create a six-week window during which the electors must be chosen and certified, leaving little time for errors or challenges to the results. The delays inevitable in widespread voting by mail would make it difficult or impossible for some states to meet the Dec. 14 deadline, even without challenges to the results—which are certain this year if the election is close.