Mexican officials have called it the “Berlin Wall”; others have compared it with the security barrier that Israel is building on the West Bank.

Many say that, whatever you call it, the fence is no answer at all to the question of what the United States should do about immigration, with 20 years of failed “crackdowns” behind it.

That unanswered question has begun to dominate US politics, to an extent easy for other countries to overlook. It crops up in schools, pensions, healthcare, big questions of national identity, and, again and again, in the need for money. The US population is expected to increase at a rate that European countries would find unimaginable. In 1990 it was 249 million; now it is 298 million; by 2050 it is expected (by the US and the United Nations) to be 420 million.

That astounding jump — of 70 per cent, or 170 million, in only 60 years — will surely affect how the US handles itself in the world.

At the very least, it will make the US more introverted as it struggles to digest that social revolution.

Almost the last thing the House of Representatives did before the Christmas break, after a furious two-day debate, was to pass a very tough version of a Bill to fight illegal immigration. In a battle that split the Republicans, President Bush put himself on the liberal side of his own party, arguing that the US needs these workers more than it needs to shut them out. He wanted a “guest-worker” programme, to allow some to stay legally — a “carrot and stick” approach.

Republican leaders ignored him, a small sign of his weakness after a bruising autumn. But in choosing to wield just the stick, they have made a risky move. They will appeal to the border states, whose hospitals and schools are overcrowded with Mexican immigrants. But they may alienate the Hispanic vote across the country in November’s mid-term elections, as well as businesses, unions and churches.

The centrepiece of the House legislation is the building of the wall along a third of the border, through the deserts of California, Arizona, New Mexico and Texas.

The law would also make it a felony to live in the US illegally (at the moment, it is a crime to enter illegally, but only a civil offence to stay).

It would hold businesses responsible for checking that employees are allowed to work. And it would demand that illegal immigrants are held in detention centres until deported (at the moment, in a much- derided policy, they are told to turn up for hearings, which most of them ignore).

When the Senate gets back after the break, it will have to work on its own version, to be reconciled later with that of the House. It is expected to be marginally more liberal, with the inclusion of a “guest-worker” clause that would allow migrants to take mainly low-skilled jobs for six years before returning home.

But neither version will satisfy Democratic leaders who say the Bill is a sham: that it would fail to get rid of illegal workers, or to give them a chance to become legal.

Unsurprisingly, Mexico loathes the legislation. President Fox, who called the barrier scheme “shameful”, urged Americans to remember that many of their ancestors came to the US as immigrants.

He argues that it fails to recognise the contribution that Mexicans make to the economy. So does President Bush, who, as Governor of Texas, was aware of the economy’s need for low-skilled workers and who has backed the “guest-worker” programme.

The heat of this row comes from the new pressures of those knocking at the door. A report last month by the Centre for Immigration Studies, an independent think-tank, found that 7.9 million people moved to the US in the past five years. That is two and a half times as many as the record 1910 wave of European immigration, driven by Italians, Austrians, Hungarians and Russians.

The centre’s report says there are now 35.2 million foreign-born people living in the US, between 9 million and 13 million are there illegally and more than half of those are thought to be Mexican.

About 1.2 million Mexicans and Central Americans were arrested this year trying to get into the US.

This is hardly the first crackdown, nor the first to try to hold employers responsible.

The most recent was in 1996, and President Reagan tried energetically in 1986, offering amnesty to those who were already in the country and, again, employer sanctions.

It didn’t work. There was no will to enforce the sanctions, which would have crippled many businesses.

If the same proves true again, it will show that the changes taking hold of the US are too big to be held at bay by a wall. Adolfo Aguilar Zinser, Mexico’s former Ambassador to the United States, told a University of California conference in the late 1990s that “Mexico is in the United States . . . It’s not a question of labour markets any more. It’s a question of two societies that are overlapping.”

He is right. The superpower’s attention is bound to be consumed by the huge challenge of remaking its society with a Hispanic character — and with many, many more people.

It does not yet know what to do. If the record of the “crackdowns” of the past 20 years is a guide, then building a wall will look like a monument to anxiety — but it will not look like an answer to the biggest challenge the US has faced.

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