Chances are Donald Trump has mastered the art of “under-promise, over-deliver.” The man best known for his confidence, even arrogance, has a surprisingly humble attitude toward his own debate skills. The pressure not to make a critical mistake was already great enough on the top candidates participating in next week’s Republican presidential debate, but it has increased exponentially with the emergence of the outspoken business magnate as the GOP front-runner. The other candidates have their hands full developing a “Trump strategy” to counter what the leading contender may do — a task made all the more difficult when Trump himself...