Neither Obama nor Clinton are sponsored by unions. Their major sponsors are Wall St, universities, SV and Hollywood, i.e. typical wealthy democrats.

http://www.opensecrets.org/PRES08/contrib.php?cid=N00009638

Maybe you were thinking about Bernie Sanders?

Unions don't donate directly, they make independent campaign expenditures through Super PACs. Look in the Independent Expenditures tab in the page you gave me.