THERE is no phenomenon more important in shaping American policy, at home or abroad, than the huge tide of immigration rolling in across its borders.

In the home stretch of George W. Bush’s presidency, a vast amount of attention is still given to the analysis of his inclinations. As the imminence of his departure begins to drain the interest from that exercise, the same attention will be transferred to Hillary Clinton and John McCain. That is wilfully blind. The scale of immigration that the US is facing is so large that the subject will come to dominate all of its politics.

The past fortnight has given us a taste of the future. Aerial photographs of the sea of people demonstrating in Los Angeles on Saturday suggested the crowd was half a million strong. There were 300,000 in Chicago on March 10, 50,000 in Denver, 20,000 in Phoenix, and 10,000 in Milwaukee.

Their immediate target is the Bill to clamp down on illegal immigration, which the Senate will begin considering today. The House of Representatives has already passed a version that would make it a crime to be in the US illegally; impose new penalties on employers who hire illegal immigrants; demand that churches check the legal status of those they help; and put up a security fence along a third of the US-Mexico border.

The row will dominate the summit in Cancún, Mexico, on Thursday between President Bush and his Mexican and Canadian counterparts.

The grounds for this protest have been building for years. The Pew Hispanic Centre estimates that there are more than 11 million illegal immigrants, and that nearly four fifths are from Mexico or other Latin American countries. What is new is that this huge community has found a voice. Many have relatives and children who are US citizens, giving them a sharp sense of legitimacy. Many of the 197 US Roman Catholic dioceses have joined in on the side of the immigrants. Seven months before the mid-term elections for the House and Senate, the dispute has split a Republican Party, already torn by rows over Iraq.

Bush wants to give immigrants a guest permit to stay, perhaps for three years, to do a job, particularly if it is one that Americans don’t want. They could reapply for more time, but it would not automatically lead to citizenship.

Senator John McCain, at this point probably the leading Republican contender for the next presidency, wants a Bill that would allow immigrants to apply for citizenship once they paid taxes and a fine and learned English.

Others, such as Bill Frist, the second-most powerful Republican in the Senate, say the borders should be tightened for reasons of national security.

Hispanics fiercely resent that claim. “When did you ever see a Mexican blow up the World Trade Centre?” David Gonzalez, a Los Angeles marcher, asked the Associated Press. “Who do you think built the World Trade Centre?” For the moment, Democrats are keeping quiet. As on Iraq, they don’t feel they have to do anything else to profit.

Just as Republicans began to make big inroads into the traditionally Democrat immigrant vote, they have jeopardised it.

But it is not something either party can duck. By US and United Nations estimates, the US population is expected to increase at an astounding rate. In 1990 it was 249 million; now it is 298 million; by 2050 it is expected to be 420 million.

That is a jump of 70 per cent, or 170 million, in only 60 years. The rate already eclipses the record 1910 wave of European immigration.

It is bound to make the US more introverted, with preoccupations very different from Europe’s. Those who protest against the US’s “overbearing” foreign policy should consider how hard it may become to keep it interested in the rest of the world when it faces such a revolution at home.

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