The American Federation of Television and Radio Artists and Screen Actors Guild said that they have sent out ballots on their proposed merger to the 131,000 members of both organizations.
Ballots are due back by March 30.
The mailing comes after opponents of the plan filed a lawsuit Wednesday charging SAG's board of directors with breach of fiduciary duties.
The suit also alleges that the union failed to conduct a study detailing the effects that joining with AFTRA will have on SAG's pension and health benefits.
SAG filed a motion to dismiss the lawsuit Friday, but the case is expected to be heard in federal court days before the ballots are due to be returned.
Among the co-plaintiffs suing SAG are Martin Sheen, Edward Asner and Ed Harris.
In announcing that ballots had been shipped, SAG countered with its own celebrity supporters.
The union said that more than 1,100 members of SAG and AFTRA have signed a statement of support to endorse the effort to merge the unions.
Among the supporters are Kevin Bacon, Alec Baldwin, Lewis Black, Amy Brenneman, Rosanne Cash, George Clooney, Jon Cryer, Danny DeVito, Robert De Niro, Sally Field, Jenna Fischer, Brad Garrett, Melissa Gilbert, Tom Hanks, Felicity Huffman, William H. Macy, Wendie Malick, Alfred Molina, Octavia Spencer, Stanley Tucci, Betty White and Rainn Wilson.
The leadership of both unions hailed a merger as the best way to bolster leverage in bargaining future contracts.
“I'm excited and grateful that we now have the opportunity to make this pivotal choice about our future,” SAG National President Ken Howard said in a statement. “More than ever, SAG and AFTRA members understand that being divided hurts us, and coming together in one union will strengthen us in every way. This merger will make SAG-AFTRA the largest, most powerful union in the entertainment and media industries -- and it couldn't come at a better time.”
“Working closely together, the leaders of AFTRA and SAG crafted a plan for the merged union members told us they wanted,” AFTRA National President Roberta Reardon said in a statement. “A yes vote ensures a strong future for all members who work in the entertainment and media industries. Now is their chance to secure that vision for themselves and for future generations.”