Many primary websites are now displayed on the root domain, with the www subdomain redirecting to the root domain. For example, NationBuilder's website displays on its root domain. People prefer this as a way to elevate brand recognition and to simplify URLs.
It is not possible to attach a root domain to a site on NationBuilder. It would be good to have the option to redirect the www subdomain to a nation's root domain. I imagine this could be an option when editing the www subdomain in the Settings section of your control panel if NationBuilder hosts DNS records.
Update: This is not something we plan to change. NationBuilder needs to route all traffic under the www. or a subdomain. Configuring the root domain through an A Record would would leave the domain vulnerable to a DDoS attack. While its true that NationBuilder.com configures its domain through the root domain, the NationBuilder.com domain is configured differently from customer nations. This domain is deliberately configured this way to run several services including customer nations.
This is something we will consider but does not have a timeline at the moment - currently you can use a redirect to achieve this.
However we are paying close attention to the most popular suggestions so please share and encourage others to follow and comment!