expression, godan verb (-ru)
go into opposition; turn against; side with the opposition
Used when someone switches sides or takes a stance against a person, group, or plan they previously supported or were neutral toward.
彼は突然反対に回って、私たちの計画を批判し始めた。
He suddenly turned against us and started criticizing our plan.
その議員は与党から野党へ反対に回った。
The lawmaker went over to the opposition from the ruling party.