Teachers unions have steadily amped up their political involvement: From 2004 to 2016, their donations grew from $4.3 million to more than $32 million -- an all-time high. Even more than most labor unions, they have little use for Republicans, giving Democrats at least 94 percent of the funds they contributed to candidates and parties since as far back as 1990, where our data begins.

Two organizations account for practically all of the contributions made by teachers unions: The National Education Association (about $20 million in 2016) and the American Federation of Teachers (almost $12 million). Both groups -- which compete for members, but also collaborate with each other through the NEA-AFT Partnership -- are consistently among the organizations that contribute the most money to candidates and political groups. [Read more Background]