(Not sure if this should be on the History SE)
I'm thinking of something like this: say Alice is the Prime Minister, and Bob is the Leader of the Opposition. Alice does something which Bob strongly approves of, and Bob switches from being a member of the Opposition to being a supporter of the government. For example, by voting for Alice's party in the next general election (Bob might need to step down as Leader of the Opposition in this case).
Has this ever happened? I'm aware of some consensus decisions where both the incumbent government & the opposition would've done the same thing if elected (example), but not of any which "converts" the opposition into a supporter.
I am not limiting this to parliamentary forms of government. For an example, the President of the United States's opposition member could be the Speaker of the United States House of Representatives. Also, it doesn't have to be the leader. A prominent member of the opposition should suffice.