Washington — Mexico's sudden near-insolvency underlines the problem of 3,000 miles of poorly policed northern border.
The United States, with the most generous immigration law in the world, has a birthrate population increase of only about 0.8 percent a year, while Mexico's rate is still very high at around 3 percent. Now a surge is anticipated of emigrants coming over the US border into a country with 9.8 percent unemployment and up to 6 million or more ''illegals.''
The US Senate last week passed a new immigration restriction bill, 81 to 18, on which the House has not acted. The pending bill is the most sweeping in 30 years. Mexico's bulging population coincides with a threatened near-collapse of its credit structure, and the prospect of heavy migration and extraordinary strain for the inadequate US Border Patrol.
Former immigration commissioner Leopold Chapman estimated (in 1976) that ''only one of every three or four persons attempting to enter the US illegally is apprehended.''
''The US receives more immigrants for permanent settlement and more refugees than any other country in the world,'' report Elaine M. Murphy and Patricia Cancellier in a recent study for the Population Reference Bureau Inc. ''In fact the number is approximately twice as large as that received by all other nations in the world combined.'' These figures do not include so-called illegals.
Near-collapse of the Mexican economy puts a strain on the congressional effort to regularize immigration. On one side of the border is a nation of some 230 million with what demographers call ''below replacement'' fertility; on the other side is rapidly expanding Mexico, with population around 71 million and a growth rate just under 3 percent.
In the euphoria of high oil prices, which have now been reduced, the Mexican government's external debt jumped from $23 billion to $53 billion, with inflation rising from 25 percent to a present near-100 percent a year. Combined public and private foreign debt is around $80 billion. There is a large impoverished lower class in an economy where the upper 10th traditionally controls 50 percent of the economy.
The present fiscal crisis is accentuated by a drop in oil prices. The London-based magazine, Economist, characterizes the sudden near-default as an ''economic earthquake'' that shakes some US lenders; for example, one study says ''the nine biggest banks in the US have lent the equivalent of 40 percent of their capital and reserves to Mexico.''
The sudden Mexican crisis with its prospective pressure along the border has caught the pending US immigration control bill halfway through Congress. The Senate passed a bill sponsored by Sen. Alan K. Simpson (R) of Wyoming, but the House may not have time to act before adjournment.
The reform bill is in the hands of Rep. Peter W. Rodino Jr. (D) of New Jersey , who heads the House Judiciary Committee. A major feature is application of sanctions to employers who hire illegal aliens, commonly believed to be coming into the US at the rate of half a million a year. Mr. Rodino favors sanctions and got them through the House twice, in 1972 and 1973, but the Senate did not act. Now it is the other way around.
Some US border communities fear a flood of immigrants. Los Angeles County Supervisor Pete Schabarum estimates his community spends $50 million annually on illegal residents' health expenses. The US Immigration and Naturalization Service estimates annual cost to taxpayers for services to illegal residents is