George Clooney, Robert De Niro, Tom Hanks, Octavia Spencer and Betty White have come onboard as endorsers of the proposed merger between the Screen Actors Guild and the American Federation of Television & Radio Artists.

The five were listed among more than 75 “featured supporters” on the unions’ official merger site Monday at Sagaftra.org. Other high-profile backers include Jon Cryer, Jane Curtin, Dana Delaney, Danny DeVito, Jenna Fischer, Felicity Huffman, Alfred Molina and Rainn Wilson.

The website features a “statement of support” section where several hundred members have endorsed the merger.

“A merger of AFTRA and SAG will increase our bargaining strength and give us more power to safeguard our wages, residuals, working conditions and, of course, our benefits of health and retirement (pension and health),” the statement says. “Our employers operate across all types of media. We must have the power to meet them as one union with one voice.”

Ballots will go out Feb. 27 to 120,000 SAG members and 70,000 AFTRA members. Ballots will be tabulated March 30, and 60% of those voting in both unions must approve for the merger to go through.

Merger opponents have contended that the combo of unions will not solve problems in such areas as split SAG and AFTRA earnings on pension and health contributions and will not lead to increased negotiating strength. But candidates backing merger have won nearly all the seats in recent elections.

Clooney, De Niro, Hanks and Meryl Streep became involved in SAG politics in early 2008 by urging SAG leaders to begin contract negotiations as early as possible. The guild didn’t sign a deal on its feature-primetime contract until mid-2009.

The 18-page document that’s being sent to members next week was posted Sunday night on the Sagaftra.org site. The document includes a 1,000-word opposition statement and rebuttals to that statement by both unions.

An attorney for opponents issued a statement Monday criticizing the rebuttals, asserting that the unions are making assertions than can’t be backed up.

“They urge once again that the members should ‘trust’ that they will use the alleged increases in negotiating strength to negotiate contract benefits…that have been given away year after year,” said David Casselman. “Merger will not change the fact that AFTRA and SAG have been negotiating together for decades. The solution is to merge actors, not unions.”