More than 100 U.S. political leaders have ancestors who were slaveholders

A chiseled block of sandstone in the U.S. Capitol’s visitor center serves as a reminder that the home of the nation’s Congress was built in part by enslaved Black people. A bronze plaque says the stone, originally part of the building’s exterior, “commemorates their important role in building the Capitol.”

Many lawmakers need look no further than their own family histories to find a much more personal connection to slavery in America, a brutal system of oppression that resulted in the deadliest conflict in U.S. history.

In researching the genealogies of...