The lawsuit filed in U.S. District Court in San Diego argues that Sgt. Gary Stein has a constitutional right to free speech, the San Diego Union-Tribune reported.
A meeting is scheduled Thursday at Camp Pendleton to decide if Stein should be discharged under "other than honorable" conditions.
At issue is whether a Facebook page is a private communication or a public statement. Members of the U.S. military are barred from running for office and other political activities.
Stein's lawyers argue Facebook is private, although Stein's Armed Forces Tea Party page, first set up in 2010, now has 26,000 followers.
"A Marine doesn't give up his First Amendment right to free speech by being a member of the Marines," even if he criticizes the administration or political policy, said J. Mark Brewer, one of the lawyers involved. "They're seeking a negative service characterization against him, all because he posted some observations allegedly on Facebook that were politically not acceptable to the command."
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