at (one's) heels
1. Following close behind someone, often in an annoying way. I'm a preschool teacher, so I've had toddlers at my heels all day. The interior designer can't get any work done with your puppy at her heels!
2. Close to overtaking a fellow competitor. The leading candidate should be concerned about the underdog at his heels. If she does poorly on this exam, she may lose the title of valedictorian to one of the students at her heels.