Hang by a hair/thread
Definition 1 of 1
Pro Tip 1/3Precariousness Highlighted
Use it when outcomes depend on small, uncertain events conveying that the situation is highly unstable. 
He's one test away from failing the class; his passing grade is hanging by a hair.
Pro Tip 2/3Emotional Tension
Perfect for dramatic scenarios where there's anxiety over potential negative outcomes. 
After the scandal, her political career was hanging by a thread.
Pro Tip 3/3Critical Decisions
Suggests a critical decision or action could determine the direction or result of the situation. 
Their survival hung by a thread as the rescue team approached.