Public-sector unions--groups like the American Federation of Teachers or AFSCME--channel funds to a variety of political organizations, as disclosed to the Federal Election Commission. OpenSecrets, part of the nonpartisan Center for Responsive Politics, tabulates that in 2016 they donated $65.6 million to various "soft/outside money" organizations as well as to political parties and campaigns.
Of $17.7 million donated to candidates and their parties in 2016, 90% went to Democrats. The Democrats' share in the 2018 election cycle was slightly lower, at 85%.