Customer.io allows you to send single emails or drip campaigns when users take specific actions. These messages can be triggered by the user completing certain events (signing a petition, launching a campaign) or by the user entering a pre-defined segment (users who have not logged in for the last 30 days; users who have signed a petition, but not launched their own campaign).
We imagine that customers could use this tool to re-engage supporters who have not taken action recently. For these users, you could send a single reminder email or send them a series of emails reminding them about your organization and the great things they can do on your site.
Alternatively, you could use the drip campaign feature to send petition creators a series of short messages with tips about running effective campaigns.
Another service that allows you to send single emails, text messages, or push notifications is Outbound.