BOSTON (AP) — Brace yourself for what's expected to be the first U.S. presidential election conducted mostly by mail. It could be messy.

Amid the coronavirus pandemic, voting by mail in a contactless and socially distant way seems like a no-brainer. States have made the option widely available -- only 10 now require voters to provide an excuse beyond fear of COVID-19 when requesting a ballot by mail -- and some three in four Americans are expected to embrace the option for the Nov. 3 presidential election, up from one in four in the 2018 contest.

But running a vote-by-mail election is surprisi...