a (difficult, hard, tough, etc.) needle to thread

A very narrow margin within which one must maneuver in order to accomplish or achieve something successfully. Likened to the eye of a needle, the tiny opening through which thread is passed before sewing. There are a lot of complicating factors, so we recognize that it is a difficult needle to thread, but we're doing our best to ensure that the legislation benefits the people who need it the most. The candidate now finds herself facing a particularly tough needle to thread, having to negotiate the desires of several wildly different groups of constituents. Keeping the company afloat without resorting to layoffs is going to be a hard needle to thread.