I’d be very interested to see this broken down by type of union - most people in unions today are in public employee unions, not steelworkers, electricians, etc. And public employee union members (SEIU etc) have somewhat of a vested interest in keeping the $$ flowing
With respect to Federal employees here is a possible answer.
“In this instance, 35.1% of those who participated identified themselves as a Republican. 30.8% identified themselves as independents and 30.7% identified themselves as a Democrat. This is fairly consistent with national trends as the number of independents are generally increasing and the number of people who consider themselves Democrats is decreasing. In December 2011, 35.4% of Americans considered themselves Republicans–a total just below the high for the year of 35.6% reached in May.”
http://www.fedsmith.com/2012/01/12/federal-employee-preferences-would-not-vote/