Lyndon B. Johnson

A Southern Democrat who had once voted against civil rights bills, Johnson committed his party to them as president, signing the Civil Rights Act and the Voting Rights Act. He is often reported to have predicted that it would cost Democrats the South for a generation; the quote is not well sourced, but the result happened. White Southern voters moved steadily to the Republicans while Black voters became the most loyal Democratic constituency, a realignment that still structures American elections.