It was Ed Miliband, the British Labor Party leader, who posed the haunting question in last week'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 coverup 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. It was like the collapse of a bridge that had stood solid as millions of vehicles rumbled across, but ruptured with the addition of one more car. Just so, the tale of Milly Dowler produced a turn in public opinion, a galvanizing jolt to the politicians and a catastrophic event for Murdoch.

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 brinkmanship 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 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." Partisans on either side already know what they think -- whether it's a question in Britain about phone hacking or one in America about the debt limit. But 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. Kohut cites the public's slowly rising support since the mid-1990s for the federal government's role in solving problems. But that support broke after Obama pushed so hard for health care legislation in 2009, without a national consensus. By the fall of 2010, among independents, there was an 11 percentage point increase in mistrust for Washington compared to 2006, and a 26-point increase in support for the GOP's ability to reduce the budget deficit. This shift among independents was the sea change behind the 2010 congressional elections.

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.