on the fence
on the fence {adj.} or {adv. phr.}
Not able, or not wanting to choose; in doubt; undecided. — Often used with "sit".
Jack sat on the fence for a week last spring before he finally joined the track team instead of the baseball team.
Mrs. Jones has decided to vote for the Democrats, but Mr. Jones is still on the fence.
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Source: A Dictionary of American Idioms
