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.