I never quite understand why people think that the US armed forces would never fire upon revolting citizens. Such a claim ignores not only world history, but even the history of the United States. Maybe people like to believe that such events are extraordinary and don't apply to us?
People think that everyone in the military is a conservative who loves shooting guns.
There are of course a lot of conservatives in the military, more probably in two specific branches than the others. But there are also a lot of liberals in the military.
As to percentages? Couldn't really tell you. Military Times (and the various branch's respective Times) all report during election years that their polls show people favor the republican candidate. That is also the Military Times though, which I have never found to be that reputable.
I can tell you that in my time in the Air Force I probably only met two people who for sure voted Republican every time. There were a couple who never stated their political preference, but the majority of people I ever knew or worked with all voted Democrat, wanted to get rid of DADT, recognized that too much is spent on defense (who better to know how goddamn wasteful the military is than people seeing and doing the wasting, lol), and various other things.
As Madness said, it depends on who the people revolting are. The people who were diehard birthers or didn't think the POTUS was eligible and swore up and down they would not defend his orders would of course side with any revolt, but a lot of them made asses out of themselves early on after the election in '08 and were dealt with.
If it is people revolting because President Obama suspended Habeas Corpus, locked down the country in Martial Law for no reason, and was eating babies, then of course the military would not stand by him.
If it was something like we are experiencing right now with fear-mongering from the right about how the country is officially done and that are just waiting for shit to pop off? I would like to think that the majority would still obey the president's orders and defend the country and Constitution (because they would see through the bullshit that 'the Constitution was under attack.').
That is what I would like to think anyway. No way to back that up of course and there are some dumbasses who were a uniform, but I would like to think that the average enlisted man or woman has a level head on their shoulders.