WASHINGTON - It was Ed Miliband, the British Labor Party leader, who posed the haunting question in Wednesday's parliamentary debate about the phone-hacking scandal: "Why didn't more of us speak out about this earlier?"

Miliband blamed political intimidation by Rupert Murdoch's press empire: "The answer is, of course, what we all know and used to be afraid to say: News International was too powerful." But that doesn't explain the sudden discontinuity - how a story went from inaction to outrage.

The basic facts of the phone-hacking scandal were hardly a secret. A parliamentary inquiry last year showed the extent of the snooping and suggested that there had been a cover-up by the police and News International. Yet British politicians didn't take action until a sickening new fact was added to the mix: The hacking victims had included a 13-year-old murder victim named Milly Dowler.

This was the tipping point.

Maybe it's stretching things, but I see a similar inflection point in the American public's attitude about the debt-limit extension. For months, President Obama has been warning that GOP brinksmanship was dangerous. Along with every responsible economist and business leader, the president said it was reckless to hold hostage the nation's creditworthiness. Republicans kept barreling through this flashing yellow light.

But all of a sudden, the light turned red. Polling by The Washington Post and other organizations showed a sharp increase in public worry about the potential cost of the debt-limit shenanigans.

What is happening in these opinion swings that move an issue from the usual "ho-hum" to a level of concern that forces political action? I asked pollster Andrew Kohut of the Pew Research Center, who has been sampling opinion in America and abroad for decades.

Kohut calls it the "we've-had-enough factor." He explains: "All of a sudden, people realize, Holy Moly, this is really bad! Prior to that, they had looked the other way." The key change agents are the "independents" in the middle. Folks in the middle take awhile to form their opinion; when they finally do, the balance tips decisively.

Big political changes reflect breaks in the smooth contour of opinion, especially among these independents.

The lesson is that the great slumbering middle still makes the decisive difference in politics, when it pays attention. Partisan voices may seem to dominate the debate. But the changes that matter - as when the British public decides it's fed up with Rupert Murdoch's brand of journalism, or when the American public demands that politicians stop playing games with the budget - happen because people in the center get angry and demand action.

David Ignatius' email address is davidignatius@washpost.com.