Has a leader ever been so successful the opposition “switched sides”?

Question

(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.

Answer 1

Winston Churchill, no less, switched from the Conservatives to the Liberals while a member of Parliament, and later on (in 1923), re-joined the Conservatives. He remarked at the time,

"Anyone can rat, but it takes a certain amount of ingenuity to re-rat." (ref)

Churchill was not Prime Minister at the time, but he was an influential MP. And that's merely one famous example of a great number of others. For example, in the US, a number of southern Democrats switched to the Republican party starting in 1968 through the 70s and 80s in response to the Republicans' "Southern Strategy".

Answer 2

There are numerous cases of congressional members in the US (both House and Senate) switching parties. The most common form is for the congressman to change to independent and caucus with the former opposing party, but there are instances when they've made the committed switch from D to R or R to D. This is often, but not always, something that happens when the original party falls into the minority, or when a change of party affiliation can affect the control of the chamber.

Answer 3

Party-switchers are rarely a result of the opposition's resounding success, and are usually a result of their allies' resounding failure (at least in the switcher's eyes). Exactly what constitutes a "leader" is debatable and would be pretty hard to find, but in the United States I found a few examples from the modern era of party leaders who switched sides.

John Bohlinger was the Republican Lieutenant Governor of Montana, became a Democrat in 2013. Bohlinger is probably the closest to what you're looking for, having been elected on a bipartisan ticket. While I can find no definitive reason why he switched parties, it's likely that his close affiliation with Democratic Governor Brian Schweitzer played a large role.

Tom Butler was the Democratic Majority Leader in the Alabama state Senate, became a Republican in 2017.

Beth Fukumoto was the Republican Minority Leader in the Hawaii state House of Representatives, became a Democrat in 2017.

Russell Peterson was the Republican Governor of Delaware, became a Democrat in 1996 after endorsing Democratic Presidential candidates in '88 and '92.

Ronald Reagan, possibly the United States' most famous Republican, was a Roosevelt Democrat until 1962. Reagan was a pretty minor player before switching parties, so that may or may not fit the spirit your question.

There are also multiple former state governors (the highest elected office of their state) who switched to a third party, but that doesn't really count as declaring for the "opposition." All of the examples above cited as their reason, when one is given, the failures of their own party rather than the successes of their opponents.