Violent attempts to overturn an election aren’t new in American politics. After the Civil War, the Ku Klux Klan launched white-supremacist insurrections all across the South to stop Black people and their allies from voting. And 150 years ago, President Ulysses S. Grant and Congress responded to those vigilante attacks with a groundbreaking law. Known as the Ku Klux Klan Act of 1871, it still protects Americans from political intimidation today.

This week, the Klan Act was cited in a federal lawsuit aimed at those involved in the Jan. 6 attack on the Capitol. Filed by House Homeland Security Chairman Bennie Thompson (D-Miss.), the lawsuit accuses former president Donald Trump, his lawyer, Rudolph W. Giuliani, and members of the Proud Boys and Oath Keepers of conspiring in violation of the Klan Act to prevent Congress from certifying Joe Biden’s victory in the 2020 election.

The Klan, founded in Tennessee in 1865 by Confederate veterans, grew by 1867 into an armed paramilitary force that pledged to restore “a white man’s government” in the South. In disguises to shield their identities, Klansmen intimidated and murdered Black and White members of the Republican Party after Abraham Lincoln was assassinated. Klan violence peaked just before the 1868 and 1870 elections.

“We have just passed through an Election which, for rancour and virulence on the part of the opposition, has never been excelled in any civilized community,” South Carolina’s Republican governor, Robert K. Scott, wrote to Grant in fall 1870. “Colored men and women have been dragged from their homes at the dead hour of night and most cruelly and brutally scourged,” Scott reported, “for the sole reason that they dared to exercise their own opinions upon political subjects.”

The opposition, Scott told Grant, had declared “that they will not submit to any election which does not place them in power.” Klan sympathizers were even plotting to disrupt the vote tally. “I am convinced that an outbreak will occur here on Friday ... the day appointed by law for the counting of ballots,” Scott wrote.

Grant, elected president in 1868, had led the Union Army to victory in the Civil War. But as letters from his Southern supporters beseeched him for help, Grant realized that the Klan threatened to undo the U.S. government’s postwar efforts to create a multiracial democracy.

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The Ku Klux Klan Act of 1871 — also known as the Third Enforcement Act — made it a federal crime to use “force, intimidation, or threat” to infringe on people’s rights to vote, hold office, testify in court and serve on a jury. Targeting the Klan’s tactics, the act also made it illegal to “go in disguise upon the public highway or upon the premises of another” to deprive anyone of equal protection of the law. The act also allowed victims to sue the perpetrators in federal court.