Jordan Misra at the Census Bureau:
The availability of alternative voting methods continues to change how voters vote. Alternative voting methods include any method other than voting in person on Election Day, such as early voting and voting by mail.
In 2018, 40 percent of voters used an alternative voting method. The percentage of voters that cast their ballot early or by mail usually declines slightly in midterm elections relative to the preceding presidential election. However, the rate in 2018 was not significantly different from the 2016 presidential election.