DoD has issued a memo to civilian employees and military personnel warning them that they “are encouraged to carry out the obligations of citizenship” they must “be aware of the limitations that exist when participating in political activity”
The memo recounts policies under the Hatch Act, including links to guidance on issues such as use of social media. “As a general matter, activity is political if its primary purpose involves activity directed toward the success or failure of a political party or organization or the election of a partisan candidate,” it says.
The memo covers various other issues as well, including requests to use military facilities for political purposes, and stricter limits on activities on some types of employees.
The memo is here: http://cpol.army.mil/library/general/elections/depsecdef_06192012.html