By default, all of your One Click Politics messages can be delivered to your recipients by anyone who signs through your embedded widget or custom form.
However, many legislators simply ignore any correspondence from people outside of their districts - if they aren't a constituent, a lot of the time their opinion simply does not matter to the elected official. It is very likely that if you are creating a campaign that targets your state's representatives, or if you want a supporter's message to go only to their own representatives in Congress instead of all Congress members, you may want to turn this setting on for your campaign.
For this reason, we have the "Constituent Mail Only" option available in the Message Settings for each campaign. Turning this setting on is as easy as clicking just one button: