Margaret Sanger and other early supporters of birth control were racists, who feared that non-whites would overcome America if their numbers were not limited. So, as conservative media often claim, anyone who campaigns for expanded birth control services today is also a racist. Right?

Wrong. Call it the fruit-of-the-poisonous-tree fallacy: If something had evil roots, it remains evil. The fallacy is deeply bipartisan, infecting liberals and conservatives alike. And it’s poison to honest political dialogue, which should acknowledge history without blithely assuming that we’re prisoners of it.

Our latest example comes courtesy of American Federation of Teachers president Randi Weingarten, who told 1,400 teachers at the AFT’s summer conference on Thursday that school-voucher proposals are “only slightly more polite cousins of segregation.”

As Weingarten pointed out, early advocates of public dollars for private schools tried to use such plans to resist court-ordered desegregation. In Prince Edward County, Virginia, most notoriously, white officials closed the entire public school system and then created whites-only private academies, paid for with public funds.

She’s right about the history, just as birth-control critics are correct about Margaret Sanger. But it hardly follows that Secretary of Education Betsy DeVos and other school-voucher proponents today are racist, any more than Planned Parenthood is. ...