With 10 days to go before ballots go out, high-profile thesps have started lining up on both sides of the proposed merger between the Screen Actors Guild and the American Federation of Television & Radio Artists.

On Thursday, Sam Jaeger (“Parenthood”), Seamus Dever (“Castle”) and “Modern Family” cast members Ed O’Neill, Jesse Tyler Ferguson, Julie Bowen and Eric Stonestreet appeared in videos on the pro-merger http://www.sagaftra.com web site.

On the vote-no side, former SAG president Alan Rosenberg and Scott Wilson — winner of SAG’s Ralph Morgan award for service — joined about 30 antimerger protesters in front of SAG and AFTRA headquarters.

Meanwhile, attorneys for merger opponents, headed by David Casselman of Wasserman, Comden, Casselman & Esensten, have been tussling with SAG attorneys over the materials to be included in the ballot materials.

Casselman disclosed Thursday evening that he and SAG attorneys had reached a tentative agreement on the Opposition Report that will be sent to members. That document — limited to 1,000 words — was not required as part of the ballot materials but the SAG and AFTRA boards OK’d its inclusion last month.

Pro-merger forces have contended that SAG and AFTRA will have more clout as a combined union — a contention that been disputed on numerous occasions by Rosenberg, who served two terms as SAG prexy from 2005 to 2009. He continues to insist that AFTRA’s leaders have been too accommodating to employers, and that a merger will put those leaders in a dominant position.

“This merger is being done so that no one will stand up to the employers,” Rosenberg said. “It’s a lie to say that we’ll be stronger by merging with AFTRA. The same leaders pushing this are the same ones who made a deal last year where we gave away first-class travel and had no advances on new-media provisions.”

The merger proposal will be sent to 120,000 SAG members and 70,000 AFTRA members, who include actors, broadcasters, DJs, singers and dancers. To pass, the referendum needs a 60% approval margin from both orgs among votes cast. SAG and AFTRA will mail out ballots on Feb. 27, with a tabulation date of March 30.

Opponents are also objecting to the lack of a comprehensive analysis of combining the SAG and AFTRA pension and health plans and to the assertions by the unions in their feasibility study.

The unions’ summary of the feasibility study noted that several hundred multi-employer pensions have merged over the past 25 years, and there is no legal obstacle to merging the SAG and AFTRA pension and health plans. It also said multi-employer plan mergers do not pose any increased risk of loss of benefits.

Cassleman disagreed.

“The crux of the problem is that SAG is representing that merger will result in no reduction of pension or health benefits,” he said. “But the truth is only the merger of the unions is on the table now. If the unions are merged, SAG members will have no further voice or opportunity to oppose a later merger of the pension or health plans, despite the highly probable negative effect that plan mergers will have on their benefits.”

Casselman also asserted that the merger proposal is also being promoted as if it will solve the problem of split actor earnings under the separate SAG and AFTRA P&H plans.

Merger backers contend that combining SAG and AFTRA will make it easier to combine the plans as a first step toward resolving the problem that performers face in making contributions to the separate plans and then not meeting the earnings qualifications.

“But the undisclosed truth is that the merger proposal does not even offer a solution to that longstanding problem,” Casselman said. “The merger plan will codify the split earnings dilemma exactly as things stand today.”

SAG First VP Ned Vaughn, a leader of the Unite for Strength faction that has dominated guild politics in recent years, responded by asserting that merger opponents don’t have the backing of members.

“The same crowd keeps trying the same failed tactic, and it seems like they’re getting desperate. When they can’t get the support of members, they hire a legal gun to accomplish what they can’t get done at the ballot box. Four years of election returns have made it clear that members want nothing to do with their divisiveness,” Vaughn said.

He also questioned Casselman’s expertise.

“I’m amazed their lawyer thinks he knows more about the security of our pension and health benefits than a half-dozen of the country’s top experts, who all agree that merger is a great step,” Vaughn said. “One thing he obviously doesn’t know about is the disastrous effect having separate unions has had on working-class actors. That’s why the real experts — our members — have made it so clear they want SAG and AFTRA to merge.”