Need, like dare, is sometimes called a "semi-modal", in that it can be used like modals (eg can, may) but can also be used as a normal verb (eg like want). So The balance need be swayed only slightly is parallel to The balance can be swayed only slightly (the meaning is different - this is just showing the syntax); whereas The balance needs to ... More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.