When Donald J. Trump’s campaign recently began to enroll “election observers” to monitor the vote this November, the news media reacted with shocked surprise. Politico called the move “unprecedented in a presidential election,” and others predicted that it could lead to voter intimidation, or worse, at the polls.

But we don’t have to guess at what partisan election observers might look like. There’s a long history of such behavior at American elections, much of it quite ugly. At an 1859 election in Baltimore, “challengers” snatched ballots from voters, sparking citywide riots that left two dozen beaten, four stabbed and eight shot. The Baltimore Sun complained that many citizens considered such violence “ordinary incidents of a popular election.”

They were right: Intimidation and violence were a regular part of the electoral process. We’ve come a long way, for sure — but with angry partisanship and even political violence on the rise, it’s worth asking what happened, and how we can avoid the same thing today.

The Constitution sets out no rules for how Americans should vote, so throughout our history political parties have improvised. As America expanded the right to vote during the 1800s, most states adopted mass-distributed paper ballots, but left it to the parties to print those ballots and get them into the wooden ballot boxes sitting in town squares or saloons.