Like most changes in a nation's way of life, this one came unannounced, unlegislated, and without fanfares. It came in the form of the front page of last Wednesday's New York Post and it showed a puffy-faced teenage girl wiping away a tear from her left eye. But this was not a picture of just any tearful teenager. The headline read "Chelsea's Heartache", and the subhead "Romance ends for `stressed' First Daughter".
The news that Chelsea Clinton had broken up with her boyfriend Matthew Pierce and had shown up in tears at Stanford University's campus medical centre complaining of stress marked something more than the end of a teenage romance. It marked another chapter in the corruption of American innocence and it was executed by a tabloid newspaper owned by Rupert Murdoch.
Until last week, the American press had an unwritten but strictly observed pact with the White House. The terms were clear and straightforward: Chelsea Clinton was off-limits.
The children of political leaders are never wholly out of the public eye, of course, no matter how assiduously their parents try to protect them. And there have been presidential daughters in the White House before - Caroline Kennedy, Julie Nixon and Amy Carter among them - in recent decades. But those earlier young women grew up in a less intrusive age, however controversial their fathers may have been.
Nevertheless, for six years, Chelsea has been protected from media intrusion by a generally honoured agreement. She went to a local fee-paying school, but no paparazzi followed her there each day. She hung out with her friends in Georgetown and the suburbs, spent weekends at their houses and invited them to hers. She shopped in the local malls with her mother. All of it, more or less, without prying lenses and media attention.
Still, from time to time, she would appear at family photo opportunities and occasionally at White House functions. There were even stories, invariably anodyne, about Chelsea's dance classes, her affection for the White House cat, Socks, etc etc. The pact ended when the Post broke the taboo, quoting "friends at Stanford" who said Chelsea had checked in to the medical centre on Monday last week "complaining of shortness of breath and clutching her forehead".
"She was rubbing her temples and was breathing quickly," the Post source claimed. "She said she was under a lot of stress, then she mentioned she'd just ended a long-term relationship and that it was causing her a lot of distress. She kept saying `I'm not adjusting well', over and over." What happened next was the familiar process of media recycling, into which even those who disapproved were drawn. That Chelsea may now be treated as fair game by the media is the latest example of this year's gradual and very painful convergence between America's tabloid media culture and the much more high-minded broadsheet dailies.
Indeed, America is a two-nation media culture, whose bestselling publications are the so-called "supermarket tabloids" such as the National Enquirer and the Star, which are sold in their millions at check-out counters. Papers such as the Washington Post and the New York Times traditionally see themselves as having nothing in common with these scandal sheets but those days may be ending, as tabloids such as Murdoch's Post increasingly bridge the gap between the supermarket papers and the up-market mainstream press.
Two weeks ago, the National Enquirer ran a story which suggested Chelsea's health was collapsing because of her rows with her father over his lies about Monica Lewinsky. The same week the Star announced that Chelsea believed her parents would divorce after her father left the White House. By last week, even the White House press corps was asking Lockhart about the story.
The White House may protest about declining media standards, but the truth is that the White House itself has not baulked at playing the Chelsea card when it felt it needed to.
Through absolutely no fault of her own, Chelsea Clinton's very existence is a counterweight to the storm-tossed domestic crises of the Clinton marriage. Her apparent normality, niceness and goodness are an asset not just to herself but to her parents. And her parents are sometimes willing to exploit that asset, just as they exploit their pets. It is no accident, for instance, that in this year of swirling White House sexual scandal, Hillary Clinton should have just published a book, Dear Socks, Dear Buddy, about the Clintons' cat and dog.
The most prominent example of this strategy came last August, on the day after Bill Clinton's fateful grand jury testimony in the Lewinsky scandal. August 17th was probably the worst day of the president's career, his marriage and his tenure of office. The next morning, though, the embattled First Family had to leave for a delayed vacation.
As the Clintons emerged from the White House to brave the cameras, Chelsea Clinton conspicuously held her parents together, holding her mother by the left hand and her father by the right. When they arrived in Martha's Vineyard a couple of hours later, Chelsea's role was even more prominent. Unbelievably, this normally reticent and almost invisible teenager was suddenly the central icon of the moment. She lingered much longer than usual in front of the cameras, smiling and shaking hands with party loyalist welcomers, before departing with a wave.
The drafting of Chelsea to play such a crucial role in what the columnist George Will memorably calls "the grotesque pantomime of domesticity that the Clintons perform in public" could not have been a casual act. Instead, it looked like what it was, a completely deliberate and political use of the Clinton daughter to help to save a presidency which, at that moment, appeared close to disintegration.
It is not hard to see why Chelsea is such an asset in a world in which people form judgments about public figures on the basis of little hard evidence. Opinion polls show that Chelsea Clinton is very popular with the American public in a way which her father cannot rival.
People feel sorry for her, admire her for her apparent optimism and her young-adult dignity. She seems like the kind of daughter that any family would be proud of, and she is therefore a crucial riposte to those who claim that the Clintons are private monsters.
THE striking comparison here is with Prince William. In Britain, the public insists on projecting on to the next generation of royals the hope that the 16-year-old prince will grow into a better person with a less troubled life than his parents'.
Whether, on a personal level, Prince William really merits these hopes is completely unknown. Whether he can bear the weight of them remains anybody's guess.
In many ways Chelsea Clinton plays a similar role. Though she never speaks in public, and few who really know her speak about her either, Americans seize on signs that she plays an important role in reconciling and counselling her parents.
"I love my dad. I understand. I can cope," she is alleged to have said as the president began to confess privately, then publicly, to a more truthful version of his relationship with Lewinsky.
Will this be a story with a happy ending, in which Chelsea pulls the Clintons together, restores her parents' relationship, copes with the break-up of her romance with Matthew Pierce and continues to grow up into the adult daughter that any parents would be proud of? The reality is that no one, not even the supermarket tabloids, knows.
Or perhaps the real guide to the real Chelsea is a story told by Clinton biographer David Maraniss.
The year is 1988, and Governor Clinton of Arkansas is deciding whether to run for his party's presidential nomination. Clinton and his chum Mickey Kantor are on the lawn of the governor's mansion, debating, when the seven-year-old Chelsea runs out from the house and asks about the family's summer holiday plans. Clinton, stumblingly, confesses he might not be able to come because he may be running for president.
"Well," replies Chelsea, "then Mom and I will go without you."