Whispering-gallery-mode (WGM) microresonators provide a high-performance platform for measuring single nanoparticles and viruses, as well as large molecules. However, there is still room for further improving their sensitivity and detection limit, towards their theoretical limit. Here, we present a new method that enhances the performance of WGM sensors based on the mode-splitting method. We show that scatterer-induced mode splitting is significantly enhanced in a rotating resonator. This enhancement originates from the different Sagnac frequency shifts that the clockwise and counterclockwise optical fields in the resonator experience due to the rotation of the resonator. Our approach, combining Sagnac shift and mode splitting, provides a new route for enhancing the coherent optical sensing of nanoparticles with single-particle resolution. In addition, our results shed light on the studies of, e.g., topological or optoacoustic effects with rotating devices.