When Donald J. Trump promised to turn Muslims away from American shores, they wagged their fingers.

When he mimicked a journalist for his lifelong disability, they tsked-tsked.

When he mocked the mother of a valorous soldier killed in combat, they threatened to walk away from him.

But Republican leaders never did. They justified his behavior, they minimized his offenses, they excused his insults.

So why this?

Why did a decade-old three-minute video provoke a sudden revolt by party officials against their nominee, an uprising that could very well destroy their chances of taking the White House?