Joe Biden’s endorsement by the National Education Association, America’s largest union, puts the likely Democratic nominee in this year’s presidential election on a potential collision course with some of his party’s most loyal supporters: African American and Hispanic voters who support charter schools and other school choice options.

The unions, of course, provide more than votes; they’re also among the Democratic coalition’s most reliable sources of campaign contributions and election manpower. From 1990 through 2016, for example, “practically all of the contributions made by teachers unions” to candidates and parties — at least 94%, according to the Center for Responsive Politics’s OpenSecrets.org — went to Democrats. In 2016 alone, OpenSecrets reports, this amounted to more than $32 million, “an all-time high.”