at someone's heels
Also, on someone's heels. Immediately behind, in close pursuit. This idiom is used both literally, as in Jean's dog was always at her heels, and figuratively, as in Although his company dominated the technology, he always felt that his competitors were on his heels. This idiom appeared in the 14th-century romance Sir Gawain and the Green Knight. The expression is sometimes intensified as hard on someone's heels or hot on someone's heels. Also see on the heels of.