The Hatch Act[1] restricts the ability of Executive branch employees to engage in partisan politics. The Smith Act[2] prohibited executive employees from advocating the overthrow of the US Government, although I think it's been found unconstitutional.
Note that I'm 98% sure that it's still illegal to advocate the violent overthrow of the government, so civil servants should take the Smith Act overturning with a grain of salt.
[1] http://en.wikipedia.org/wiki/Hatch_Act_of_1939
[2] http://en.wikipedia.org/wiki/Smith_Act