NTEU has a good run down of what political activities you can engage in (when and how) in their "Status Call" newsletter.
A Quick Breakdown:
What If I’m Not On Duty or At Work?
If you are not on duty, in the workplace, or wearing a uniform or in a government vehicle, as a federal employee, you are free to participate in many political activities, such as:
expressing opinions about candidates;
putting out campaign signs in your yard;
contributing money to TEPAC, candidates or political organizations or attending fundraisers;
campaigning for or against candidates, including phone banking or canvassing;
registering and voting;
registering and encouraging others to vote;
driving others to the polls; and
attending political rallies.
The Hatch Act allows you to be a good citizen and be active in the many important issues affecting this union and our country. If in doubt about what you are permitted to do as a federal employee, check with your NTEU National Field Representative.