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.

More Information about the Hatch Act is on NTEU website.  

Or download the Status Call which explains the Hatch Act a bit more. 

StatusCall--Mar-Jun-Hatch Act.pdf