Taken together, these two oddities explain why immigration in America is such an explosive topic, and why Congress is unable to pass any new law regulating the flow.

The recent collapse of bipartisan negotiations in the Senate on a new immigration bill probably marks an end for this year of the attempt to impose some order on what many Americans see as out-of-control illegal immigration.

What split both parties and ultimately doomed the law were President George Bush's proposals for an amnesty for 9 million of the estimated 11 million illegal immigrants already in the United States, and a new program to admit an extra 400,000 temporary "guest workers" every year.

The House of Representatives recently passed a much tougher law involving serious penalties for employers who hire illegal immigrants and the construction of a 1,100-km fence along much of the Mexican border.

But with Congress now in recess, that is probably dead too.

There is probably neither the time nor the political will for the Senate to have another go at the issue before the elections due this fall.

What this is all about is Mexicans.

The United States, contrary to local belief, does not have a particularly high proportion of recent immigrants compared to other industrialized countries.

No more than one person in eight is foreign-born in the U.S., considerably less than in neighbouring Canada (where the ratio is one in five) and not much more than in large European countries like Germany, France or Britain.

But nowhere else has so many illegal immigrants, nor so many who are unskilled workers, nor such a high share from a single country.

Mexican nationals make up the great majority of the "undocumented workers" (illegal immigrants) in the U.S. economy.

Their large numbers and high visibility give rise to paranoid fears among some longer established Americans that the United States is becoming a de facto bilingual country.

They also stir a wider concern that this large, vulnerable workforce of illegal immigrants is deliberately maintained by employers as a way of keeping the wages of unskilled workers down.

The language issue is largely a red herring: Most newly arrived Hispanic families have become fluent in English by the second generation, just as previous waves of immigrants did before them.

But the argument that illegal immigrants take jobs away from many equally unskilled native-born Americans, and drive wages down for the rest, has never been convincingly refuted, even though it remains politically incorrect.

It's not that native-born American high-school dropouts "won't do those jobs." They just won't do them for $5 or $8 an hour — or at least, a lot of them won't.

Many poor Americans simply have no choice, however, and end up working long hours in miserable jobs for half the money that an unskilled French or German worker would earn for doing the same work.

One of the most ridiculous myths of American political discourse is the argument that the U.S.-Mexican frontier is too long to police effectively and humanely.

Here is a country that has landed people on the moon, and that currently maintains an army of 140,000 soldiers in a hostile country halfway around the planet, claiming that it cannot build and maintain a decent fence along the Mexican border.

Instead, we have been treated to a 30-year political charade in which little bits of fence are built in the traditional urban crossing places, thus forcing illegal Mexican immigrants out into the desert where many of them die — but enough still get through to keep America's low-wage industries fully manned.

Living right next to Mexico, a country where a large proportion of the population lives in Third-World conditions, does create a special immigration problem for the United States, but it is far from insoluble.

It has only remained unsolved for decades because powerful economic interests in the United States, with great influence over Congress, do not want it solved.

All the other business that has been so earnestly debated in recent weeks in the United States Senate — quotas for guest-workers, amnesties for long-resident illegal immigrants, and so on — is just the political cover that is needed to keep illegal immigrant labour plentiful and unskilled wages low.

Gwynne Dyer is a Canadian journalist based in London whose articles are published in 45 countries.