Military.com has this report about a U.S. Marine who, while in uniform, sang the National Anthem at a political rally for Republican presidential hopeful Donald Trump. It notes past incidents:
The Marine Corps has previously pursued disciplinary action against Marines who violated directives governing political activity. In 2007, officials investigated Marine veteran Adam Kokesh when he appeared at an anti-Iraq War rally wearing the Marine camouflage utilities uniform.
And more recently, Marine Sgt. Gary Stein was demoted to lance corporal and received an other-than-honorable discharge in 2012 after starting an online Tea Party group and a Facebook post in which he disparaged President Barack Obama and said he "would not follow all orders from him." In Stein's case, he also broke a military law prohibiting troops from disparaging their commander-in-chief.