The roots of Mr. Trump’s approach date to before his election in 2016, and he advanced his plans throughout his term. But his strategy for casting doubt on the outcome of the 2020 campaign took shape in earnest when the coronavirus pandemic upended normal life and led states to promote voting by mail.

From the start, the president saw mail-in ballots as a political threat that would appeal more to Democrats than to his followers. And so he and his allies sought to block moves to make absentee voting easier and to slow the counting of mail ballots. This allowed Mr. Trump to do two things: claim an early victory on election night and paint ballots that were counted later for his opponent as fraudulent.

The United States Postal Service, after coming under the leadership of a Trump ally, Louis DeJoy, made several cost-saving moves that severely slowed mail delivery rates and prompted broad concern about mail ballots arriving on time.

In the Senate, under the leadership of Mitch McConnell, the majority leader, Republicans blocked Democratic efforts to get more money to states so they could buy more sorting equipment to count the huge influx of mail ballots faster.