In the upcoming election, she is considered the main rival to the incumbent mayor.

rival 是名词，意思是"竞争对手"或"敌手"，指在某个领域或竞争中与另一方对抗的人或团体。这个词来自拉丁语 rivalis，原指"住在同一条河对岸的人"，后来引申为"争夺同一资源的人"。