When tweeting via a social share prompt on a NationBuilder page, the NationBuilder system calculates the total character length of the tweet -- including the link (eg. "http://www.example.com/volunteer?recruiter_id=123456") and returns an error message if that is greater than 140 characters. However, twitter no longer counts links towards the 140 character limit, so posting that to twitter directly is no issue. It seems that the validation/block is on the NationBuilder side of the integration. In addition, If the integration can be tweaked so that the 140-character limit validation only checks against the tweet content, there is an older issue will no longer need to be resolved: The character counter in the social share settings counts the number of characters in the tweet but does not take the link into consideration.
All public and control panel locations where you can tweet now use the updated character limits.