Let us, in this season of happy tidings and joyous illusions, cast our minds forward to the summer of 2007. For the second time in less than a decade, the House of Representatives is drawing up articles of impeachment against a president, upon which the Senate will vote. This time, however, the Democrats, not Republicans are in charge on Capitol Hill, and the man in the dock is not William Jefferson Clinton, but George Walker Bush. And this time the point at issue is not squalid but inconsequential acts of fornication in a White House side-chamber, but rather weightier matters: abuse of power, reckless manipulation of intelligence, and a war entered into on false pretences.

Believe this, some will say, and next I will be arguing that Santa Claus is for real. But the scenario is no longer as far-fetched as it was, after the revelation by The New York Times last week that Mr Bush personally authorised the National Security Agency (America's far larger, even more secretive version of our own GCHQ at Cheltenham) to conduct eavesdropping on US soil against US citizens without a warrant.

The disclosure has provoked an almighty row. The White House says the snooping was justified by the extraordinary nature of the "war on terror". Nonsense, contend critics who say the measure is a blatant over-reach of presidential power, and a gross violation of civil liberties. As for us foreigners, we may be forgiven for wondering what the fuss is about.

Why is it that Americans are uniquely obsessed with government wiretapping? Yes, almost every country outlaws the practice, at least without a court-issued warrant. In reality, most of us probably share the "I'm shocked, shocked" attitude of Louis Renault, the world-weary police captain in Casablanca, when he "discovers" that the odd spot of gambling occurs in Rick's Bar. And if a slight bending of the law nips a terrorist outrage in the bud, then so much the better.

Here in the US, however, it's not so simple. In this most legalistic of nations, the dispute involves great principles of the US constitution. Even more important, it raises the ghost of Richard Milhous Nixon.

Mention "White House eavesdropping" to an American of a certain age, and his mind instantly goes back to Watergate, and to the plumbers, dirty tricks and enemies lists that were the subplot of the scandal. Illegal wiretapping featured large in the Abuse of Power article of impeachment approved by the House Judiciary Committee ten days before Nixon resigned.

Nixon denied wrongdoing to the end. By contrast, Mr Bush is utterly unrepentant about his recourse to the NSA, which by law is supposed to focus exclusively on foreign electronic surveillance. He insists his action was perfectly legal under the emergency powers granted him by a panicky Congress after the 11 September attacks. In short, as John Dean put it, Mr Bush "is the first-ever President who has willingly admitted to an impeachable act". And as Mr Nixon's White House Counsel, Mr Dean knows a thing or two about such matters.

Now Mr Bush would say his wiretapping is qualitatively different. Nixon went after his political enemies; the sole targets of the NSA have been suspected terrorists, enemies of all America who might be plotting a repeat of September 2001; surely a small loss of civil liberties is a price worth paying for that.

The problem is that we are being asked to take this President at his word, never the most reliable of commodities. Then there is the shameless scare-mongering by this White House, and its cynical playing of the patriotism card. Who ever leaked details of the NSA domestic spying, it maintains, was giving succour to the enemy. Mr Bush is famously obsessed with leaks. So why might he not wiretap suspected leakers and their accomplices in the press?

Equally specious is his claim that existing procedures are too slow and cumbersome to deal with the terrorist threat. A special court, set up under the post-Watergate Foreign Intelligence Surveillance Act of 1978, exists to authorise electronic surveillance of targets in the US. The law even has a 72-hour retroactive clause; in other words the government is entitled to read a target's e-mails and listen in to his phone calls for three days without prior authorisation. In short, a perfectly good tool already exists.

George Bush is not Richard Nixon. The latter was driven to his excesses by the paranoid conviction that his political foes (who controlled Congress in 1974) would get him unless he got them first. Mr Bush's motives are different, mirroring the impatience of a corporate CEO accustomed to having his every instruction instantly obeyed.

But the 43rd President has one over-riding belief in common with the 37th. This is that the power of the executive must be protected, and if anything expanded. The most unashamed proponent of this theory is, of course, Dick Cheney, who, as chief of staff to Nixon's successor, Gerald Ford, saw with his own eyes how, after Watergate, power seeped from the White House to Congress.

"The world we live in demands a strong, robust executive authority," the Vice-President argued this week. "Either we're serious about fighting the war on terror or we're not."

But what goes around comes around, even for imperial presidencies. Mr Bush's approval ratings have lately plunged to almost Nixonian levels.

So will imperial overreach lead to the nemesis of impeachment? As of now, no chance. Suppose, however, that Democrats gain control of the Senate and House at the mid-term elections in November. According to the polls, it could happen. In that case, four years of unanswered questions would spill out into Capitol Hill's hearing rooms. And impeachment, now just a dream of Bush-haters and others on the farther shores of liberalism, could become the drama of 2007.