"It's an environment like nothing I've ever seen," said Democratic nominee Gretchen Whitmer, a former legislative leader, who has held an edge in state public opinion surveys.
I think "held an edge" here means "had an edge". The last part is saying that the former legislative leader took the lead in state public opinion surveys.
But I'm not quite sure if my understanding is correct because I've never seen the phrase "hold an edge".
The full source.