No person employed in the executive branch of the Federal Government, or any agency or department thereof, shall use his official authority or influence for the purpose of interfering with an election or affecting the result thereof. No person occupying a position in the competitive service shall take any active part in political management or in political campaigns, except as may be provided by or pursuant to statute. All such persons shall retain the right to vote as they may choose and to express their opinions on all political subjects and candidates.