A: Are you sure of your success in this election?
B: No. I can't bank on their support.
B: Because they are used to sitting on the fence.
E.g. You can bank on his help.
E.g. You can always bank on me if you need money.
E.g. The Berlin government is banking on the Olympics to save the city money.
sit on the fence：保持中立，保持骑墙态度
E.g. He thought that it was better for him to sit on the fence during the quarrel.
E.g. Sooner or later you'll have to come down on one side or the other; you can't just sit on the fence.
be sure of：确定
E.g. She is not sure of his answering her letter.
E.g. Be sure of your facts before you write the article.