Half to Three-Quarters of Them Would Be in Jail

"During the early 20th century, strict nonpartisanship was the professional norm. The overwhelming majority of officers even refused to vote since this required them to think of themselves as partisans for the time it took to cast a secret ballot. As late as 1976, 55% of the higher ranks (majors and above) continued to identify as independents.
"Vietnam marked a decisive change. With leading Democrats challenging the Cold War consensus, party politics began to threaten key military interests, and many officers began abandoning their detached stance. With the political rise of Ronald Reagan, the top rank of the officer corps moved from 33% Republican in 1976 to 53% in 1984. By 1996, 67% of the senior officer corps were Republicans, and only 7% were Democrats—the basic pattern continued through 2004."

Bruce Ackerman in the Los Angeles Times warns against partisanship among military officers.