Try impossible, due to UCMJ.
And I'm a GREEN suiter, too. You think I can ever join a FReep?
Whatever you career can handle, is what we always say...
Of course you can. Just not in uniform. Best not to appear to speak for the military either, even if only by identifying yourself as a member. You can't speak contemptuously of the POTUS, but you can politely argue your side of an issue.
There once was a tradition that military did not get involved in politics at all, not even to vote. That day is long gone. There are limits, but they are fairly broad.